[Kube]

The seat may have to be removed and the tracks cleaned thoroughly. Occasionally a simple lubricant application won't do the trick.

If you see the latch move - that's a good thing. Is it worn along the bottom edge or straight across? If it appears straight, then most likely the adjustment between the catch and latch is poor.
You can experiment with moving the catch one way or another a bit at a time or, as I tend to do, climb in the trunk with a flashlight and have a helper close the trunk. You'll quickly see what the issue is and whether or not an adjustment will fix it.