Some of the rear hatches from mid-90's Chevy Blazers and GMC Jimmy's has bad
hinges on the top that caused the hatch to creep upward over the years.  You
can identify this problem by looking at the top edge of the hatch where it
meets the back roof line - if it's flush with the roof line then no problem
and you can stop reading right here, but if it's raised above the roof line
the hinges are seperating and the hatch is literally "creeping" upward.
What happens is that as the hatch creeps upward in the closed position it
places pressure on the locking mechanism at the bottom until it finally
jams.  To open it you need three people working together - the first person
stands on the rear bumper with both hands on the top edge of the hatch
pressing downward with their body weight, the second person needs to press
the release button, and the third needs to pull on the lift handle of the
hatch.  Pressing down releases the upward pressure on the latch from the bad
hinges, and pulling out on the handle along the bottom edge of the glass
hatch will open the hatch when the release button is pressed.

Good luck - Jonathan