Check your logs~
Look at where you start up world and zone from, make sure you have a /logs/ subdirectory. If you don't, create it and start up again.
Read the logs, the error on why your mobs aren't spawning is going to be in there. Once you understand why it isn't spawning mobs, you can look at svndir/utils/sql/ or whatever else to find patches that solve the issue, like the mentioned above unique_spawn patch.
Note: your bash shell will also output the errors too if you watch that when you enter a zone, but logs are a good habit for when your system crashes and such.