When trying t download a new version of minecraft (For example 1.8, or snapshots) It takes a long time to load. When it completes, the game doesn't launch.

My launcher opens, I press play. The load bar moves really fast to the end, where it then says "Downloading 1.8.jar". It takes a while for the blue bar to go to the end, when it gets there, the blue bar goes away, yet the game does not launch. I have retried up to 10 times, yet nothing changes.