How many items are you currently watching? You should be able to watch up to 400, including all active and ended items.

Be sure you are looking at your entire list, not just a subset. If you are at or near 400 total items, try deleting some old or ended items to make room for new items.

Some Watch List issues can be helped by signing out of eBay, restarting your browser or updating your computer, and then signing back in. If there are any pending windows updates or if your browser has not been updated in a while, odd problems can crop up and various features can stop working correctly.

If you still have problems adding items to your Watch List, try another browser to see if that makes any difference.