Viagra won't work if her mind's not in the game, erectile dysfunction is just as much about the mind as it is the penis. Being on HRT may or may not reduce libido but it will generally change the way the body responds to sexual and sensual stimulus - it sounds like the old sexual dynamic is no longer the same. As iKate says, you may need to find a new way to play, try to pleasure her the way you'd like to be pleasured and see if that makes a difference.
Also, the medication has a long lead time. The longer she has been on it the longer she'd need to go without it for the effects to reverse and there will be a lag time when she takes it back up again.