Garage Door Won't Close, Reverts near bottom, Why?
Garage motors have a torque sensor to minimise the crushing risk. this means that if the motor is under too much load, it will automatically stop.  My guess is that your track is bent, or dirty and is creating too much friction for the motor. 

you can test the opening and closing force by releasing the motor (pulling the wire to bypass the motor) and manually opening and closing the garage door. (do you see that its difficult to open at any point?)

the solution is to clean the tracks with a cloth and apply some grease... now garage vendors dont like you greasing your own track, so check your warantee first... but you will be amazed what a good greasing will do to the tracks... 

regarding the type of grease to use, i would check with the motor manufacturer, the reason they dont like it is becuase grease can attract dust if you are in a dusty area, typically I simply apply a petrolium based grease on a clean track and 99% of the time the problem goes away.