Do you live in an area that has sand burrs, cactus or other sticky plants that keep popping your lawn mower tires? You try pulling them out but the normally break off and leave the sharp point inside the tire which penetrates the tube. Or the air comes out through the new hole that is created in a tubeless tire.
You can either keep fighting this problem or you can change to resolve it. There are a couple of ways to solve it and getting all the pointy things out of the yard is not really a good option.
You can either use tire slime inside the tire or you can install a tube that has slime in it. Personally, I like the tube with the slime in it because it gives you another layer of rubber as a buffer against those pointy things.
Tire slime is a thick liquid with particles in it that stops up those holes caused by items that penetrate including nails. It seals all the tiny holes and creates an airtight seal. The slime rolls with the tire and spreads itself out over the flat surface of the inside of the tire.
Flat lawn mower tires are a pain because it takes a bit of work to get them off the rims. So whichever solution you use, this will help solve your flat tire problems for a good while.