Apparently if they come under a lot of vibration, like transportation in a vehicle, the little clips can work loose. They pack theirs 15 to a box without the plastic bag and that keeps them tight enough so that the little clips don't work loose. Tap the clip portion on your tripod leg after removing it. This will make sure it's fully seated. Make sure all your pulls are dead straight.
Some people experience blank negs.
Kodak produces crappy holders, pre-broken straight from the factory. I got one while at the workshop but kodak was nice enough to fedex me another one overnight. Waste one sheet. If you're too skinflint to do that, get another hobby. AND, when exposing check the envelope after pulling the darkslide by swooping your finger along it.