My guess is they are not Play Store games and in that case not all games, or apps for that matter, are compatible with all devices. Especially when you download games from the web, they may install or appear to but that doesn't ensure they will work on that device.
As an additional security tip, it's the wild wild west out there on many of those app sites, even if they have an app to provide the apps doesn't mean they're safe. Try to stick with trusted places like Amazon App Store, Galaxy Store and Google Play. Other manufacturers may have sites as well and those are likely safe but just any old place is a crap shoot.