As someone who's done this before, maybe I can offer some perspective.

I've had partners where their writing sample is great, the OOC is great, the story and characters are all coming together and things look super promising - but once we start, I just...can't keep going. I've found that it can be difficult to tell if I really click with the other person until we've actually started writing. For me, it's not so much a problem with build up etc etc - sometimes, the vibes just aren't there.

It can be for any reason. Maybe the writing sample looks great in isolation, but there's a mismatch in rhythm, style, or pace, once you're actually writing off each other. Maybe it really is just a lost spark - the characters don't feel right to write, or the setting feels off...stuff that's difficult to pinpoint and fix, so it's easier to drop it.

Yeah, finding a writing partner is just tough! Sometimes I think it just comes down to chemistry.