Well, you can jerk the spawn rates up to 50, last I knew, so I would experiment with raising the numbers until you are satisfied with the amount of z’s in your world.
Personally I really hate dynamic spawn worlds, and I’ve always viewed that setting as more of a challenge for the particularly brave players and less of a realistic rpg.
It is possible that you are encountering a bug, but I would try using a high spawn ratio and static spawn setting to double check before you delete and re-download the game.
After this, I would just clear it and download again.
Also, what version are you playing?
If this is an experimental, then bugs are kind of expected.
If it’s an old version, I would definitely recommend downloading the most recent version.