The major reason why babies suddenly stop using signs is because parents do. Pretty simple right? A baby looks up to his parents as a role model of what to do, so if you stop, they stop too. The best way to keep your baby signing, is to continue signing along with your baby. You can’t really expect babies to do as they are told and not as you do!
Other times, babies stop using signs because they’ve lost interest in saying the word – they’ve moved on. That doesn’t mean they wont come back to the sign – because eventually, so long as you keep using it, so will they. Babies also sometimes forget what the sign is when they learn to say words. This is pretty normal and not always something you can prevent from happening. Rather then force your baby to sign, if they prefer not to, you should celebrate the development of their spoken vocabulary. In the long run, speaking is what the aim of signing is really all about.
Other times a baby can become distracted by other tasks such as learning to walk or crawl, playing with new toys, and so forth. Important milestones can often put a baby off signing temporarily. So if you want your baby to continue to sign, then just keep signing without him, eventually your baby will regain his interest.