@ruth4tea,
Are you using the app or logging in on a computer? I'm using a computer and in the last few days have entered several items to my watch list with no problem. If your watch list is set to display items by Time: Ending soonest, it may be way back on the list.
If you're using a computer and you click on the Add to Watch List button from an item listing, in the upper left just above the photo a blue banner should open saying added to watch list.
If you are trying to add an item from the results page I do not see the watch button on my machine.
One thing you might try if you shop/search on your device a lot, would be to clear your temporary internet files and cookie cache, then restart your device. Too many files and cookies can interfere with some site's operations.
"THERE IS NO SUCH THING AS FOOLPROOF, BECAUSE FOOLS ARE SO DARNED INGENIOUS!" (unknown)
