#1) The most common cause of this is incorrect API details for the captcha feature (if captcha was enabled). So double check the captcha documentation and verify that the correct information was entered.
#2) The 2nd most common cause is page caching. Please empty any caching on your site. You can try to exclude the page (containing the payment button) from caching.
#3) The next item to check is to make sure there is no conflict happening on this site:
Please post the URL of the page containing the payment button so I can see it in my browser. That can help me find some clues about the issue on your site.