Not sure if it's the same problem but on our server we had the problem that no hostile mobs were spawning (since 1.7.4, normal spawning before that) except by mob spawners. In server.properties difficulty was 2 and spawn-monsters was true. Eventually we found out that in the level.dat under Gamerules there was an entry doMobSpawning which was for some reason set to false. Changing that solved our problem.
Stefan Schlichting
Not sure if it's the same problem but on our server we had the problem that no hostile mobs were spawning (since 1.7.4, normal spawning before that) except by mob spawners. In server.properties difficulty was 2 and spawn-monsters was true.
Eventually we found out that in the level.dat under Gamerules there was an entry doMobSpawning which was for some reason set to false. Changing that solved our problem.