It could be from many different reasons, usually the case is the phone not having enough ram. So it's either you make sure this is the only thing that runs in the background, or play it on a different platform, the purchased game could be also downloaded to a PC / MAC / LINUX without additional pay, as much as you want.
99.9% of PC's have more than 2 GB's of RAM available, you need much less, but that's the benchmark, just in case.