I had one missing and ordered a new one.
If they're in properly you might have trouble getting them out. The plug has a long tail with little fins to stop them pulling out once inserted. Possible you'll break them trying to get them out to glue them back in! Probably would just give them a bit of a tug to check they're in properly and then just keep an eye on them each time you fill up.I’m going to glue mine. Not waiting for them to fall off.