What could cause Unity Ads to crash after a couple of reward videos?

I have a 3D game where the player can revive by watching a revive video, but the thing is that the game crashes after a couple of revives when the reward video is about to play. But this only happens on an HTC phone. If I try to reproduce the problem on a Samsung galaxy all is well. I suspect the problem is low RAM on the HTC phone, but any ideas on what else can cause a game to crash when a reward video is triggered on a specific phone?

Thanks :)