World won’t delete

My son has bought the How to Train your Dragon world and has been using it without issues until recently. In the last few days he cant create a new world from it. The icon says ‘downloading’ but it’s not downloading. If he clicks on it, there is no create icon, just a greyed out box. If he logs in on other devices it is active, but he wants to be able to use it on his own iPad. We can’t fix it. I tried to delete parts but don’t want to tinker too much in case I make it worse!