No Spawn of WitherSkeleton and Blazes in the Nether

I have install on a FTB-SkyFactory3 Server spongeforge-1.10.2-2477-5.2.0-BETA-2777 and now do not spawn witherSkeletons and blazes, blazes spawn arround his spawner but thats all… Skeletons and WitherSkeletons dont spawn!

SpongeForge is the only one Plugin that i have installed after the ModPack by FTB.

If i delete sponge it works with the spawn of this Mobs.
sorry for my bad english! I hope any can understand and help me fast.