So Jupiter awakens her senshi powers in Act 31, Mars in 23, and Venus in 46. I can't for the life of me remember if Mercury had an episode? It seems wrong if she didn't, but maybe it was slipped somewhere in an episode that doesn't state that in the title.

If she never did, was there an explanation/mention?

I'm not 100% sure about the act in the manga, but in the anime she does. The episode where she writes lyrics for a song of a man.

"The Power Explodes! Ami's Song From the Heart" in Japanese
"Heartfelt Melody" in the English dub

I found this on wikimoon,  "Sailor Mercury's powers were eventually upgraded by Neo-Queen Serenity and she reached her "Super" form along with the other Sailor Senshi near the end of the Infinity arc. "

And in PGSM she awakened her senshi powers in act 29.

Eugh, I didn't read which section this was in and answered for these three franchises. ¬.¬

I saw a mention of it being on Act 29, but the moment was so small compared to the other senshi I wondered if they actually meant it as a power up (because there was no mention about it either, it was literally just her gaining some kind of telekinetic water control over her tambourine for a few seconds and then Luna said something about 'nice technique' and that was that). Usually these moments are huge for the other senshi and they make a big deal about it, so yeah. I feel kind of cheated   

At least Ami got her own go to the dark side arc. That's something, right? XD

Maybe after returning from Dark Mercury Ami discovered her powers awaken?My memory of PGSM is a bit foggy.

she did unlock her powers, if i'm not mistaken when she got back from being evil or around that time, no?

need a pgsm rewatch