It seems I can only loot bodies until I have my first encounter with a baddie (wolf etc). After that, they don't show up as having anything to loot. that includes Zapruder and Wilhemmena.

You can only loot a body if it has something to loot. Once you take everything, the option to loot — or even to put something back on their corpse — is gone. If you can't loot a body, there's nothing left to take.

I talking about trying to loot them while there is still something to loot. For example, I wanted to pick up Zapruder's camera. I was waylaid by a wolf which I killed. Then I continued on to Zapruder's body. Nothing. It was as if it had already been looted (which it had not).

If that's the case, then it sounds like a glitch. Did you search the ground nearby? Did you check whether Virgil wandered over and perhaps picked it up for you?

If neither of those solve the issue, then have you already installed the UAP? If not, a link to it can be found in the stickied thread at the top of this board. I strongly recommend you install it and then start over; trying to use unpatched savegames with the patch can cause unexpected issues down the line.