Mobs don't spawn on Pressureplates/upper Block Halfslabs

So me and my friends built a slimefarm on our private server (hosted on my PC) and we get no single slime ball out of it. Chunks are slimechunks, we got over a stack during the building process. The floores are made out of Halfslabs.
After that we finished our endermanfarm where the Enderman are supposed to spawn on pressureplates.

What we expected to happen was getting tons of slimeballs and endless xp, but what actually happened we didn't get any slimeballs or xp yet.

I started the world up in v1.7.9 and both, the slimefarm and the endermanfarm work properly. You can see on the screenshots where they are supposed to spawn

Steps to Reproduce:
1. build a slime/enderman farm or any other farm and try letting mobs spawn on pressureplates/halfslabs
2. see what happens