I don't know what you're seeing on the actual site itself when you try this, but if it just fails without comment and goes back to the original state it was in before you started typing, this might be an Apple thing. On my Pad I occasionally get a situation where I spend several minutes on a post, and when I go to post it, it simply fails without explanation. I have since found I can back button to repopulate the box with what I'd written, which is nifty. Then I can copy and paste it into Notes, close and reopen the browser, re-post what I'd tried to before, and it usually works. Nothing like this ever happens on my Windows machine. 😎
There's another issue where, for some reason, one single word in your post will prevent the post from posting. When that happens, you get a dialog box saying something along the lines of the post you made is no longer there, or available, or something like that. When this happens, you have to repost your content one paragraph at a time until you figure out the offending word, at which point you just change the word and then it's fine. It's probably an Invision issue because that exact same thing happened on the old board as well.