The only thing I could think of would be easier trip. We talking pike or walleye or what here? Next time out, carefully inspect the baits on the non-laker trap and see if there are any signs that it got hit and the fish spit it because the flag didn't trip. Sometimes they are funny like that, especially walleye and bass. They'll eat the bait and sit there with it in their mouth and not move, and if they feel resistance they spit it right back out. Ever go running to a flag only to have one that you are running past trip and take off too? The fish was either staring at your bait and the sound scared the bait into moving and the fish hit it, or he had it in his mouth and the sound made it swim and trip the flag. I've seen it a bunch fishing for crappies with tipups... its to the point where if I have them out and I am hole hopping, I try to walk right by my tipups and give a good stomp next to each one. It's worked before.