I play on a server with a few other people, but we want to fight the wither to get a beacon. However, no wither skeletons or blazes spawned at our nether fortress, and after looking it up I figured the best option would be to find a different fortress because that one might've been glitched. Once i got to the fortress, i found two wither skeletons, killed them, and then no others spawned. I tried again finding a different one and the same thing happened. The light level is 0, the difficulty is on hard, and nobody else is online and i still cant get any to spawn. does anybody know why this is happening and is there any way to stop it?