I see things are working again for the original poster, but for the benefit of anyone else who finds this thread who may be having issues adding items to their watch list:
In general, if you are currently watching less than 200 items, you should have no difficulty adding new items to your watch list. If you are watching more than that you may experience erratic behavior of the list; try deleting some items one at a time. If you are unable to delete items directly from the watch list, try using the Remove link at the top of the individual item page.
Your watch list items can also be seen on your Summary page if you are using a desktop browser, so if you are having a problem specifically with your Watch list page, try that page instead. Trying the eBay app is also something to consider.
If the "add to watch list" button on an item listing has been replaced by a text link or there is no dropdown menu to add items to other custom lists, try explicitly signing out of eBay, restarting your browser and signing back in.
Sometimes issues can be caused by your particular browser, so trying an entirely different browser or another computer can be an indication if the problem is on eBay's end or your end. If things work fine on one browser but not another, there may be steps you can take, such as restarting your browser, deleting cache or temporary internet files, deleting eBay cookies, disabling extensions, or updating the browser to the latest version.
