This happen frequently with my half-year-old lumia 920; I open my phone's lock and then I press f.e. Whatsapp tile on my start screen and it doesn't do anything. I can see the tile shake a bit from touching it but it doesn't take me to the app. Same applies to almost all of my apps but for example text messages, call etc. There is no other way to fix it than reboot the phone. This problem occurs very often, I have to reboot it like every other day. Is anybody else experiencing the same issue? Do you know what causes it?