Make sure not to overpack your bowl. If the shisha touches the foil, it's a bad sign and it'll burn. Remove some shisha if this happens.
Also, like others said, too much heat can cause this as well. Try to remove a coal or two and see if that helps.
Last, try adding some ice or ice-cold water into the vase, that should help some.
Over here we only smoke Nakhla, and it's a great smoke every time