I finally figured out why enemies stop spawning, and how to make them start spwaning with the same frequency.

I'll use the skeleton example/

On the bridge, the max chain you can get at once is 52. There are Skeletons, and Skeleton Defenders. They are named Skeleton A-Z, and Skeleton Defender A-Z. When you kill all of both of them, A-Z. They stop spawning. If you go 2 screens away from your current screen (think Kingdom Hearts), they respawn. I reccomend holding R2.