Sometimes I can't pick up loot inside houses. I then have to go outside the house and run around for a bit and then run back in and try to pickup the loot. Going outside the house and then going back in usually works.

This is also for rusty trucks and jeeps. If you can pickup the loot around the cars you have to run away from it and then come back. Or try to go to another house, pick up an item and then go back to the rusty car and pick up the item you want.