When I click 'play', it'll say 'Checking For Minecraft Assets', and when the bar gets to 100% it'll change to 'Installing Minecraft Assets'. Then the launcher will close and in a blink open again. That happened to 2 packs I installed today and tried to play (Mo' Creatures + Morph; Hack/Mine). I've no idea why this is happening. With packs I already installed (Example: Aether II - Genesis of the Void) they run just fine. I did try rebooting the computer. I really need help with this, since I want to play Hack/Mine again so badly. Thanks for the answers!