Thanks so much for playing! I'm really glad you enjoyed the game :)
About cow spawns, there's a 50% chance for them to appear each time you return to the surface. So technically, you can "force" spawns by resurfacing more often. If you're running low on leather, try unequipping everything (so you don't use up pickaxe durability), dig down about 20 cells, then return to the surface. Repeat this a few times and you should see plenty of cows show up.