If you're not on TRT then using DHT gel will suppress natural testosterone production over time. This is one reason it may appear to "quit working". It is true that DHT does not aromatize, and it also has some aromatase-inhibiting activity. However, androgens do contribute directly to some suppression of the HPTA, though less than estradiol.
There are other possible reasons for diminishing effects. For one thing, there are many reports of hormones in flux providing benefits which fade when things settle down. Unfortunately the reasons are not yet understood.