Jeff, sometimes they don't trigger, you may have set it too high. I normally set mine real low to the ground and otters have triggered it. I learned this through trial and error. But I did have a similar expierience were I had deer tracks all around my camera but no deer. I had my camera out this year for the entire month of august and got nothing on it, seemed odd.
Yeah I've noticed this also, and I have had bait taken with no shots. However, in the great swamp, the bait was never touched.
Jeff