2 things could be wrong:
1: Server render distance could be less than 10 chunks. (old MC bug.)
2: The mob cap is shared with everybody on the server, so if there are already 170 mobs or w/e near other people you will see few spawns.
Also, you need to be 24+ blocks away from the spawning surface (you didn't specify) and preferably less than 32.
Each dimension (Overworld, Nether, End) has its own mob cap, So if problem 2 is affecting you, and you are doing a mob trap for EXP, then try the End.