I've often found in the past that it took 2-4 tries with a new catpcha code to get it to go through. Seemed like there was a time out issue involved too - take too long composing a message and the first catpcha expires and a second needs to be tried.
Last 6 months or so it seemed to be behaving better and goes through on first try for me though.
I ALWAYS recommend copying your message text to the clipboard before trying to submit ANYWHERE in case something goes wrong. That way you don't have to retype everything (for example: if eBay throws you to a "we are experiencing technical difficulties" page)
If 3 or 4 captcha attempts fail. Try starting the message send over (paste in the text you saved), and try again.
If still fails, other than clearing cookies and browser cache and trying, and then as a last resort trying a different browser, I got nothing else.
