I find orgasm can elusive and I am not on HT just a radical prostatectomy so far.
I think a big chunk of it is in the mind, I can have weekends where we are like teenagers and everything works fine without pills or potions (several times too!!) Then no explanation I struggle to get it up and then if I do I can't orgasm before I am knackered and my wonderful fionce has gone to sleep!!
I have been through this cycle a couple of times now so it can't be anything "physical"
The only explanation I can come up with is once the "thought" of failure enters your head you are screwed so some how you have to keep positive and definitely don't pressurise yourself..