Check that
- /gamerule domobspawning is true
- spawn-mobs=on/spawn-animals=on in server.properties
- worlds-entity-spawning-disabled does not have the world listed in server.properties
- ticks-per-entity-spawns > 0 but not too high and ticks-per-entity-despawns not too low in server.properties
- difficulty > 0 in server.properties for monsters to spawn
- no plugin is disabling it