If your baby was successfully using a sign, but stopped suddenly and you have no idea why, then it’s likely due to becoming preoccupied with another milestone. This doesn’t mean they won’t ever use it again, it just means they have shifted their focus and priorities for the time being. Your baby might be learning to climb onto the couch, trying to walk faster, or even to learn to crawl.
However, more often than not, losing a sign is due to lack of use. This can happen due to inconsistent signing on your part or failing to model the sign each time you say the word. If you get lazy about signing, then your baby will get lazy too. You have to lead by example – it’s the best way to get the results you want.
In this case, you should just continue to do the sign and stay consistent. On the other hand, if you stop signing just because your baby has, your baby will drop the sign from use permanently. Your baby will almost always take the lead from you, so continue to model the signs if it is something you want your baby to continue. When we taught my son Holden the sign for telephone (a motivating sign for him) he would do it frequently, asking to touch and play with the phone, or even answer it. After a while though, he lost interest, stopped doing it and it was dropped from his sign vocabulary. It wasn’t until sometime later that we noticed, so we just reintroduced the sign and eventually he “re-learned” it. After the second time, it became a permanent part of his vocabulary and would use it whenever the phone rang or notice my wife or me talking on the phone. I suspect that had we not brought the sign back to his attention, it would have been forgotten for good.