Rear derailleur shifting

[OakLeaf]

My first guess would be that the cable is way loose. If you can shift it by pulling it manually then you're probably just a full step too loose. Have you tried just taking out more slack in the cable? It should be taut when you're in your small cog.

Another possibility is that it's hanging up somewhere. Did you replace the housings, or only the cables? Does the cable pull smoothly through the housings?

Did you change gearing, or just put on a new cassette with the same gears? You shouldn't have to touch the L or H screws, but if you changed your low gear, you'll need to tweak the B screw.

I'd put the L screw back where it was, actually ... especially if you took your pie plate off. You don't want the chain going into the spokes.