Unless I am mistaken I believe that once a mob is spawned from a soul shard it obeys "vanilla" mob rules. Meaning you are probably to far from the spawner and the mobs are despawning...
I understood that they didn't..
All my Tier 5 mob spawners work when they're at y30, and I'm at y190 above them, and my understanding was it doesn't matter that you're in the same chunk, i'm still more than 128 blocks away so thus should instantly despawn, but they don't.
it is listed on the wiki that they spawn mobs when players are near or not. so even if you're all the way across the map they still spawn with a chunk loader, kinda silly if you can't do anything with them.