serpentine belt keeps falling off

so i was pulling out of the driveway a week back and my serpentine belt fell off. checked everything and it turns out the knob that holds my tensioner in place broke off. so i got a new one and put everything back together. well the serpentine belt fell off again. i not to up to date with car mechanics and all that jazz but i know enough to know that you shouldn't be able to push the belt down more then a 1/4 of an inch when everything is in place. but i can push it down at least an inch and that is with the new tensioner. so i returned the old one and put the new one in. same problem still occurs. anyone have any ideas as to what i can do to solve this problem? thanks. my first post also.

There will be some movement with the serpentine belt system. The spring loaded tensioner takes up the extra movement.
Check all the idler pulleys and pulley alignment.
If the belt slings off after replacement, double check your installation. If you miss a groove, the belt will walk off the pulley.