You should be able to watch up to 400 items total, active and ended.
Check to see how many ended items you have on the watch list. If you have many (387?), try deleting some of those to make room for new active items. Try viewing your Watch List using a browser, too. Make sure you are not limiting your view to a particular category, but seeing the entire list.
If you are still having problems, try signing out of eBay and signing back in -- that can sometimes help with Watch List issues.
