Only a few times have I had a child forget their word from the day before. And it was always only the day after they were first introduced to Key Words. 

If it happens, the first thing to do is take that word off the ring and blame the word, saying something about it not being a good enough word. (Set it aside and toss it after the child is away.) Then try to be sure they’re really talking about something they’re very interested in. And that they’re talking long enough about it to conjure up a vivid mind picture.

I never had a child forget a word if they had already once remembered it. So once a word stayed on the ring after the second day, it never came off. 

But if it happens that a child begins to stumble over several of the words on their ring, I’m guessing it would probably be that occasionally one or a few words they didn’t immediately recognize were left on their ring. Running into those words had probably been unnerving — confusing them, so that they began forgetting others. 

In this case, I would tell them we need to clean out their collection. You could do this by laying all words out on the floor/table and asking the child if they can find any words they know. Have them tell you what it is as they hand it to you. Then you toss rest — again saying something about to the effect that words that don’t belong sometimes get on the ring. But it’s no big deal — let’s get a better one today. 

Then, as above, try to be sure the word they’re asking for is important enough, and give them plenty of time to talk about it. 

Please just keep in mind that we’re not trying to teach them as many words as possible. We want them to collect words meaningful to them, so that they 1) not only practice associating combinations of letters/sounds with meaning but 2) they become confident they can work with print.  And leaving words on their word ring the child doesn’t recognize undermines both goals. (If this persists, then just wait and try this activity a few weeks or months later.)