If you are using the eBay app, you should be able to delete items from the Watch List by using the "edit" menu to reveal check boxes in front of each item on the list, which you can then check, and then tapping the trash icon in the upper right should delete the item(s).
If you are using a desktop browser and the old "All Lists" page, there seems to be a problem deleting items from that page. For the moment, do not use that page to delete items, particularly Watch List items.
Try deleting items from the individual list (Watch List, Wish List, etc) that the item is on, or from the individual item page itself. Try deleting items one at a time instead of in batches.
For Watch List items that cannot be deleted from your Watch List, try deleting those items from this "All Buying" page instead:
https://my.ebay.com/ws/eBayISAPI.dll?MyEbayBeta¤tpage=MyeBayNextAllBuying
If the Watch List item does not already appear on that page, open the individual item listing and add it to the Watch List again, then delete it from that "All Buying" page. That should delete the item from the Watch List.
