My machines breaked down when I tried to operate it, and after checking it I found that observer cannot give out a signal when being pushed or pulled by piston & sticky pistons. The machine was built in minecraft bedrock edition beta 1.15.0.52, at that time it can work perfectly. 

This bug not only happened on windows 10, but also take place on android platform. I wondered if it is a common bug on bedrock edition or, maybe coders are trying to change the feature of observer?