Items are unable to be picked up sometimes at seemingly random times. I came across a pistol (FNX 45), dropped a few things to peck it up, and when I was unable to I took it into my hands and then saw it in my inventory (Red Sweater) so I unequipped it. I started running to my next looting area but noticed my shiny new pistol was out of my inventory. I ran back to get it back and repeated for a few tries before deciding to post here. Similar occurrences have also happened with any item you can 'take into hands'.