It's not really a problem. It's pretty common. Check the humidity, as the enclosure may not be humid enough. The gecko may be going into a shed, as they tend not to stick as well when they're going to shed.
Otherwise, just watch the gecko, it may start sticking again. I've heard more people worried about a crested gecko not being able to stick to glass instead of not being able to stick to plastic, so it may just be a glass thing.
Most people that I've heard of with the concern, just keep watching the gecko. It may start sticking. One thing I know one person did was soak the gecko in lukewarm water daily, but I'm not sure why or what it did.