Ah!
The contrary computer syndrome attacks again.
That syndrome is, every so often, something it has done a thousand times suddenly doesn't work. You try and try but...... nothing. So, you ask someone else to come over and take a look and maybe figure out what the problem is and as they stand there watching you do exactly the same thing you just did ten times without success, Poof, everything works. Every time you try it again it works just like it is supposed to.
Your helper, thinking you must have screwed up saunters off back to their computer and so you try just one more time. TILT! Nothing. It's back to its same troublesome ways.
Something you might try the next time you try to enter a large post.
Before you try to post it, do a "Select All". Then, hit the COPY option. That will store it in the temporary memory. Then, when it doesn't work, erase maybe the bottom half of the text and try again.
There is a good chance, just to show everyone else your an idiot, it will enter the partial post.
After it is entered for all to see, click on the Modify button to edit it.
When it comes up ready for editing, go to the bottom of the text and "Paste" your stored copy of the whole post. Erase the top part of your newly pasted post down to the place where your now old posted text ends and enter it.
If your lucky, it might just change the post to show the whole thing. Or Not.
Computers seem to get a kick out of pissing off their users.