Mobs will not spawn in dark areas.. I had the difficulty to HARD and 20 minutes pass.. no mobs in the mob grinder... Im not sure if they will spawn or not..
Comments 3
Please force a crash by pressing F3 + C for 10 seconds while in-game and attach the crash report ([minecraft|http://hopper.minecraft.net/help/finding-minecraft-data-folder]/crash-reports/crash-<DATE>-client.txt) here.
"/gamerule doMobSpawning"? Mob cap reached? Distance from player? There are lots of things that could cause this, and you'll need to provide more information.