This is actually something I've learned more recently, especially with women, is pretty common. Reaching orgasm during sex often requires a lot of focus on feelings (physical, mental/emotional) and being present - ADHD brains can struggle staying present which then leads to less orgasm frequency during sex. I personally have had issues with this and have had partners with similar experiences - the best way I was able to help them was to be very vocal and engaging to keep them with me in the moment (talking in their ear, asking questions, etc) . One of my friends recently noticed that she's enjoying intimacy far more now that's she's being treated for her ADHD.