パズタイ！脳トレパズル・頭脳ゲーム

I was tapping and sliding on suspicious spots but it wasn't responding, so I got stuck. Then, after watching an ad for a hint, I tapped the spot I thought was suspicious again, and sometimes it responded. What's up with that? It happens a few times when it's my phone's issue, but most of the time it's not my phone, which is what made me irritated.