Why doesn't my mob grinder spawn mobs?

Created a 50x50 cave with a water shaft in center. The mobs are pushed up my a tripwire/repeater/stickypiston mechanism.

When the cave was at bedrock it spawned, now the top is at y:13

There are a couple of problems you could be having. As has been stated in comments above, very likely many of the mobs are spawning in other areas. Also, if you have moved up 13 levels you have greatly increased the places where mobs can spawn. If you're on bedrock, the area on the surface (assuming you're under land, and not water) is considerably smaller where mobs can spawn, than if you're up 13-20 more meters.

The second issue, is that mob caps are computed globally on a multiplayer server, so if someone pops up on the server, and starts up a mob grinder, basically all the mobs can be spawning in their areas, and not in yours. You have to out compete them on spawning space. 50x50 may not be very large for a well established server.

Finally, keep in mind that most mobs only move if they are with 32 meters of the player. So, water pads like you have are of limited utility if you are far away: