Try clicking again on all the feedback entry fields -- the buttons, the comment, the stars. Sometimes if you do not click on one of the selections, the "leave feedback" button remains gray and is not active.
Make sure there is no error message, as might be the case if there is a disallowed term in the comment section, or you are trying to leave a negative or neutral for a power seller before seven days have elapsed.
If that does not help, try using another browser or the eBay app to leave feedback.
Check to make sure the seller is still registered, and the item has not been removed from the site as well.
