Hi !
This can happen for many reasons: when it happens from Plarium they are announcing here or in Facebook Community page that the servers will be updated at a specific time. But also this can happen from your network or network provider and I noticed that can also happen when you are making fast operations in the game and the game haven't enough time to record all the operations - at that time the game will be blocked, then an error message will appear in the right bottom screen ("Blast ...error...etc").
So, what you have to do when this happen - first look on forum or Facebook Community page and see if the servers are updated in that moment and when the game running slowly or you have that error - close the page, delete history and cookies from the browser and enter again (this is good to make it from time to time even you don't have errors).
Best regards and hope my advises will help you!
Bogdan