That used to happen to me on occasion. I think it might just be a little forum glitch. I'm not exactly sure why it happens, however logging out completely and using another browser usually seems to fix it for me. Maybe that will work for you. If that doesn't work, I'd recommend logging out completely, then logging in under that little box under the banner, if you haven't done that already.
It's happened to me as well when I was using spoilers in my message.
I believe I ended up deleting them and refresh in order to make it work.
Or perhaps just restart the browser.
Keeping in mind that I was on my phone at this time.