Is it possible that you could be doing everything right, but still find that she is not interested in sex? Yes, that is certainly possible and we will show some of the common reasons on this page and how to overcome them. Just like males, females can suffer from low libido and damaged endothelial tissue.
This makes sense, if you stop and think about it, because so much of a woman's orgasm is similar to a males'. For example, none other than Masters and Johnson found that the clitoris both increases in width and length upon sexual arousal. In other words, in some ways at least, the clitoris is really a mini-penis. In fact, one could argue that blood flow is even more critical for the female. And researchers have found that Nitric Oxide plays an important role in female sexual response as well, both for engorgement and lubrication. 
So is it any wonder that decreased Nitric Oxide production and/or decreased blood flow to the vaginal area would decrease libido and responsiveness? There is strong circumstantial evidence that that is the case. For example, one study of the supplement ArginMax for Women, showed that 70+% of pre-menopausal women in the study had increased sexual desire, libido, satisfaction after taking it for just four weeks.  And guess what? Frequency of intercourse significantly improved in this group..
And it did this without increasing estrogen levels, which is important because of potential breast cancer risks. Also, the supplement significantly helped perimenopausal and postmenopausal women as well: "In contrast, among PERI women, primaryimprovements were reported for frequency of intercourse (86%; p =0.002), satisfaction with sexual relationship (79%; p = 0.03), andvaginal dryness (64%; p = 0.03) compared with placebo group." The postmenopausal group also improvment, but primarily in the area of sexual desire. (NOTE: The study was published in a peer-reviewed journal, but it should be noted that it was funded by ArginMax.)
The significance of these results is that ArginMax for Women is composed primarily of ginseng, L-Arginine and gingko. All three of these have had clinical success with males by increasing some combination of Nitric Oxide, endothelial function or blood flow. In other words, female sex drive and function is often dramatically helped by the same underlying issues that dramatically help male sex drive and function.
Is there additional evidence that this is the case? Well, there is another study showing that L-Arginine and Yohimbe significantly helped women with physical arousal.  Again, L-Arginine is the substrate for Nitric Oxide and Yohimbe increases blood flow. And, once again, both of these ingredients have clinical studies showing that they help male erectile dysfunction. Clearly, the problem is that we too often assume that that male and female sexual physiology is completely different, which is clearly not the case. Obviously, the underlying fundamentals are there for both. CAUTION: I do not recommend yohimbe for anyone as it can increase blood pressure, sometimes dramatically.
Yet another example is the fact that Kegel Exercises, the very ones that help men with erectile dysfunction, also help women with their sex life as well. Other examples abound: researchers have found that many of the same basic risk factors for male erectile dysfunction, such as high blood pressure, poor lipid profiles and cigarette smoking, are also risks for female sexual functionality. 
So the bottom line - after checking with a doctor of course! - is that most of the things in my pages for Erectile Dysfunction Supplementation and How to Improve Your Erectile Dysfunction should help the woman in your life as well.
Females and males share another common issue with libido: hormones can play a dramatic role. It is probably no surprise that testosterone, estrogen and thyroid hormones can significantly influence sexual desire in females. Let's start with the case of thyroid issues. Hyperthyroidism, an overactive thyroid, can turn your woman into a nymphomaniac, but then you probably wouldn't be reading this thread then, would you? Conversely, hypothyroidism, an underactive thyroid, will almost for sure do the opposite for the female in your life. In women hypothyroidism can lead to high prolactin levels, which is very hard on her sex life.  High prolactin levels lead to low libido, decreased lubrication and more difficulties achieving orgasm. And the problems don't just stop there: low thyroid levels often decrease both testosterone and estrogen as well.
Testosterone, as is the case in men of course, is associated with libido. One study showed that women at age 41 typically have about half of the testosterone they had at 21.  This is an even more steep decline than the typical male experiences during andropause and can definitely affect her libido, although some studies show not as much as you might think. One study shows that females can substantially optimize their testosterone through a low glycemic diet. Scientists put a group of females on low and high glycemic load diets with a washout period in between. What they found was that females produced on average 22% less testosterone on the high glycemic diet versus the low. 
Now this brings up an interesting point: guys can send their Nitric Oxide and erectile strength through the roof by eating a healthy, low glycemic Mediterranean or Low Fat Diet. Plus, by encouraging their wives or girlfriends to do the same, they will likely boost their woman's libido through testosterone and sexual function through increased Nitric Oxide and blood flow. Talk about a win-win situation!
The alternative is to eat like everyone else and slowly destroy your endothelium and hormones, curse the darkness and bitch and moan about how unfair life is.
1) J Urol, 2000, 163: 888ï¿½893
2) J of Sex & Marital Therapy, 2006, 32:369ï¿½378. (Note that a previous study showed good results as well: J Sex Marital Ther, 2001, 27:541-549.)
3) Arch Sex Behav, 2002, 31(4):323-332
5) J Urol, 2000, 163: 888ï¿½893
6) J of Urology, 2005, 174(5):1921-1925
7) Glamour, Apr 2008, p. 200.
8) J Clin Endocrinol Metab, Apr 1995, 80(4):1429-30, "Twenty-four-hour mean plasma testosterone concentration declines with age in normal premenopausal women"
9) Endocrine Abstracts, 2007, 13:P286, "Effect of Glycaemic index of the diet on salivary cortisol and testosterone levels in females"
10) Fitness Magazine, May 2006, p. 52