Have you done all the updates? The most recent was 6.3.6. If you have you may want to delete the app and then reinstall it. Just make sure you have created an account so when u reinstall the game u can start where you left off.
Mikkimat I’m also on version 6.3.4 and I’m playing on my iPhone. I have closed all other applications and made sure i had enough memory. My last resort is to delete the game and re load but I’m scared to do that so I haven’t.