There are multiple reasons why one's button code may fail however, you haven't provided enough info to help at this point. All we can do is guess. For example, it may be your web site or hosting service - the PayPal item button code is a basically an HTML Form however, some sites and/or hosting services don't play well with HTML Forms. If you're OK with providing a link to the page or site that has the code, we can take a look, else, your other option is to contact Tech Support - on this page, at the bottom, there's a link "Contact Support", on the next page you would need to login, then you can submit a support ticket.
