The rear hatch has to be properly aligned with the body of the car. If there is any misalignment, the rear striker/latch may not engage fully. When the hatch is closed, the gaps around the perimeter have to be more or less equal (the Factory Service Manual has details).
There are 3 adjustment points:
- the mounting bolts for each of the 2 upper hinges;
- the mounting bolts for the lower striker-bar (hidden under the plastic trim on the sill).
I would start with adjusting the lower striker-bar: side-to-side or front-to-back. See what happens.