Sounds like the striker has slipped down a little. That is keeping the latch from locking onto it. Try grabbing it and yanking. If it is loose then that is your culprit.

It may also be a weak/broken/disconnected spring in the latch mechanism on the boot lid (trunk lid). Next time it is open push on the latch with your finger to see if it positively returns to the latched position.

It could also be a loose latch on the lid.

Last (and most likely) the latch mechanism may be so caked with dirt, grease and gunk that it doesn't actually latch all the way. Simple visual inspeciton will show that.

What to do? Start by cleaning the latch as well as you can. If you don't want to or cannot remove it from the car, try sluicing with your favorite penetrating oil and working it with your fingers then wiping with a rag and repeating.

That will most likely "fix" it. If not, you can always look into the less likely avenues stated above!

--
Mike!