If you are using the "All Buying" page, there is currently a problem where many Watch List items do not appear on that page. If you are using the old "All Lists" page, there is a problem deleting items from that page -- do not use that page to delete any Watch List items.
Check your Watch List using this link using a desktop browser:
https://www.ebay.com/myb/WatchList
Make sure you are viewing the whole list, current and ended items, and not just looking at one category. Try deleting some ended items from the list if you are at or near the maximum (300?).
