There is a small rubber insert in the catch that pushes the latch closed. Look to see if it is dented. If so, it's your problem.

I fixed this on an '83 SD by swapping the rubber inserts from the left front to right rear. The catches are different, but the insert is the same. However, it gets dented in a different place on the left than on the right side. Swapping these inserts was only slightly harder than replacing the catches with a new one, but didn't cost anything.