Do you mount them back and forth on the same rims ? Or are they on the same rims from day one ? The beads of the tires might have been damaged when mounting.They make some stuff that looks like black tar windshield glue that will fix chunked beads, they dont have much air pressure. I use a mixture of 50% water and 50% laundry liquid soap to spray my tires and rims looking for leaks.I am sure shampoo or dish soap would be just as good. Jack that axle up so the tire can spin, dont hold back on the soapy water, around the valve stems ,cores the welded area of the rims where the center is welded to the wheel, beads to rim area ,sidewalls , tread area. then wait for it to foam up, might not be a big bubble but just foam pile on the leak. I have a hedge row of trees that have thorns on them and that is my tire ****. I put 8 oz of antifreeze mixed 50/50 in each tire, keeps the rim as wet as the day it was mounted and the thorns stay swelled up in the hole and sealed. Works good as long as I use the mower every few weeks.Sometimes one will work out and I have to plug it. This method works very well in aluminum car wheels to stop the bead leaks but 2 to 4 oz is about it you will start a balancing problem.