Is it the rear tires that are going flat all of the time and getting nails or screws in them? If so it is because the tandem rear axles are sliding sideways when you make hard turns and they will actually stand up a nail or screw into the tread of the tire. That is how most of my flat tires happen on my tandem axle trailer.
If the wheel is steel you can pound it back into shape unless it's really beat. Have you been running it into curbs? Gilla Gorilla, I've picked up a lot of nails in my back tires too. I have a love hate relationship with Slime. It works really well but make sure you get the tubeless type. The other stuff just oozes out of the bigger holes. When it comes time for a new tire your in for a nasty gooey mess.