Check that the striker is centered in the latch opening as the trunk is closed. You can observe it by lowering the seatbacks and closing the trunk from the inside. Another clue is to look for damaged finish from metal-to-metal contact. Once you verify that the striker is off center, you can loosen the bolts, move it sideways to center it, then retighten. The bolt holes have loads of slop to allow side-to-side adjustments. FYI you'll need a set of Torx sockets for this.
The strikers seem to go off center over time, probably from having stuff dragged over them and repeatedly getting whacked by the latch when the trunk is closed. When the striker is off center, it hits the side of the latch on its way down, making it difficult for the latch to engage all the way.
Centering the striker got my trunk to close properly, but I had to do a little more work to get it to open easily. When the latch hits the striker, a rotating plastic hook catches it. The trunk is held closed by a metal finger that catches a matching metal bump on top of the hook; when you press the trunk release, a solenoid withdraws the finger and allows the hook to rotate out of the way (that's the best way I can describe these parts- it makes more sense if you inspect the latch). My striker had worn an indentation in the plastic hook, which caused it to stick. I carved away the edge of the indentation with an X-Acto knife, and viola, the trunk now pops open like a champ. [thumb]