It probably depends on when you do it, and what else is going on. I had no problem whatsoever booking in April, but had some issues moving FPs around on a day that Disney opened up FPs for the Jungle Book show.
i've been tweaking my dining up until a couple weeks ago and i never had a problem with it.
i booked my FP+ on sunday and had no issues.
i was booking ADR'S and FP+ on the computer if that makes a difference to you.
I am 60 days out today and had no problem for FP on app or website. Good luck!
My ADRs are a little glitchy sometimes. I have to view details to see an accurate time on 3 of 15 of them. And it won't let me update my guests correctly on my CRT, but I think that is because of it being prepaid.