When trying to cut or add parts to a page, it isn't working. The placeholder may quickly flash "Updating" and then go back to normal, unchanged.
This typically is the result of a customization on the page. Often, from a Formatted Text and Image part of Unformatted Text part. You may need to utilize process of elimination by cutting parts off the page using the scissors icon under Site Explorer>Pages & Templates to determine which part is prompting this behavior.