Go back? How literally do you mean that? If you use the back arrow, that is why the item is not in Watch. You want to go forward, and use either the left column or the My eBay drop-down menu to find the newly added item in your Watch List.
If that doesn't work, - maybe your list is full? Only if you have 200+ items in it. In that case you need to create new lists (according to category?), move some items to these various lists, and then delete from Watch to make more space.