I had a problem with a bait stealer once. I couldn't figure out how he was doing it. I had done almost all of the above things but still couldn't catch him.  One night I just sat up and watched....he was coming every night.  I was astounded to see the biggest raccoon in the history of the world come ambling up to the trap.  I knew at once he could not fit inside the (regular coon sized trap) trap.  Well, that sucker got down on his knees and packed himself into the trap inching along until he got to the bait.  Sure enough the door came down.....ON HIS BACK!  His huge back HELD THE DOOR OPEN!!!!!  After leisurely eating the bait and licking his paws, he simply put it in reverse and wiggled out under the door.  I would have given anything for a video of that!  Hopefully this is not your problem too.  Unfortunately this one had to die of lead poisoning. So sad.