Possible issues: mechanical, obstructions, trigger, leverage, baiting technique, outside interference.
Mechanical: I'd swap out the suspect traps and test them back at the office. Use a 3/4 inch PVC pipe and as the "paw" and fire off the trap onto it. Now see how hard it is to pull the pipe out. It's easiest to put the DP in a vice to hold it in place so all that can move is the pipe. If the pipe pulls out easily, you some of your other traps and see if there is a difference or not. Springs do weaken based on use, type of trap prep done to them or you could have an manufacturing issue but if you don't test them, you can't rule this possibility out.
Obstructions: The most common obstruction, like Bob mentioned, is with the bait. However, other obstructions can occur as well based on surrounding conditions. If the DP gets knocked on it's side by deer, is set that way, or just falls over sticks, small stones or other debris can be knocked inside the trap (or for that matter fall into it if it right side up) which could prevent closing just enough to allow for a pullout.
Trigger: There are too possibilities with the triggers. If the traps have two way replacement triggers, the traps could be set off before the paw is far enough into the trap for a good capture. If the traps have a one way trigger you tend to get a deeper catch if there is nothing in the trap that can be hooked and pulled up to fire the trap. Although you said you didn't use marshmallows, it is a good example to think about with this problem. When a marshmallow gets wet, it's structural integrity changes into more of a blobish item that can be pulled on and stretched. While they still maintain some resistance, they don't have the same resistance as they do when they become slightly stale.
Leverage: I only use Sterling Lil-Griz DP's and almost had a pullout this year with a brand new trap. It was a 22 lb boar that was able to stand up and use it's size as leverage. The trap was attached to a tree with a 3 foot cable, but he was able to hook the base on some roots which shortened the distance he had to pull considerable. If he would have always tried pulling horizontally, he would have never gotten as far as being able to pull vertically (think of the root acting like the vise for testing the spring I've already talked about) as he was able to get his feet under him for additional leverage instead of behind him.
Baiting: I just covered an issue with marshmallows but again like Bob said, any type of pellet style bait (fish food, cat food, dog food, corn, shell fish, etc.) can interfere with the firing of the trap just like too much dirt or a stone can interfere with a foothold trap. My preferred baiting strategy is to install part of a tube worm over the trigger system and than use a liquid bait in the trap or a paste bait on the lip. This allows the raccoon to feel something down in the trap and grab a hold other than metal. I've had success without the tube worm as well and can't say it increases the catch when using a paste bait, but I can say it does matter for liquid style baits.
Outside Interference: While not likely, I do like to cover most of the possibilities. First possibility is that a person came by and interfered for whatever reason. Second, is that the trap could have been fired off by another animal like a mouse or chipmunk and was then "played with" by a raccoon or fox.