A lot of people have had Watch List problems lately, so perhaps there are some wondrous changes in store for us all. Like you, I also use IE, and it seems it is not as ebay-friendly as some other browsers. But I have had no trouble with my Watch List.
The only think I can think of is if your Watch List is full. That will prevent you from adding to it. If so, then you can create another list, or even many, and add those items from your Watch List to the New List, delete them from Watch, and have lots more space available. All these extra 'category' lists are on the left side of the My eBay page.
