That probably means the server is lagging or not performing properly. Our development team is still working on improving the performance and reliability of the servers.
In the meantime, a thing you can try is to use schematics to store pieces of their world that are special. Here's a video about how to do this: https://www.youtube.com/watch?v=Mqkb2Et0bZY
Of course it is better to have the world persist, but this way you will have no fear of losing special structures or creations, and you can learn a super awesome LTM feature, too!