Usually case, sometimes a stale page.
Read the captcha characters carefully and match the case exactly if it is case sensitive. Most rejections are a case mismatch or a stale page.
If the page has been open a while, reload it for a fresh captcha before trying again. A stale page can carry an expired challenge.
Before troubleshooting further, confirm the captcha's printed address matches your bar. A clone's captcha can behave oddly because it is not the real one.
