Mine does this on occasion also. on my 98 what I do is lock it, unlock it and than from the outside press down on the bottom of the hatch under the handle (to close the hatch harder) and then try to open the rear hatch keeping pressure on the bottom. It's never failed us. Those actuators are a POS and as the grease gets harder they get more finnicky to operate.
Hope it works for you otherwise the panel can come off with the removal of all 12 (2 hidden on the L-R bottom corner) and a lot of swearing as 6 of the screws are on the edge of the plastic panel accessible between the opening if you pry hard enough.
Good luck,