Rear Hatch not working.

When I press the rear hatch button under the rear wiper switch, nothing happens. I can hear the noise it makes as if it wants to unlatch the hatch, but it does not open. The vehicle is in park. On the remote, when I press the button twice, it does the same thing. It will make the noise like it wants to open up, but won't.
Any ideas as to why?

The whole gate opens without a problem, just not the glass itself.

This has been covered a few times. What usually happens is the plastic part of the actuator that connects to the hatch release mechanism brakes off. The actuator will still work but the glass will not open. Its an easier fix on a four door. Raise the rear gate and take off the entire plastic interior panel. You should have access to the actuator. Either fab up a metal piece to connect the actuator to the linkage or buy the updated actuator from gm for about $70.

Had same problem..took rear cover off..ended up being dirty mechanism..used pb blaster and is working fine now