It could be the app that you are using, that Lemmy is still going through growing pains with the new flood of traffic or your phone/PC just hates you.
You are probably going to have to track down the debug logs on whatever device and app you are using to see if there are any post errors.
Oh. Never trust any web forms on any site, especially if you are typing a big message. I know it's a pain, but selecting and copying your words is good insurance for long posts.