IIRC heart urchins don't do too well for the same reasons horseshoe crabs and sandsifter starfish don't.
They eventually get really big in the sand and run out of food to sustain themselves. I guess you could try handing it meaty food but I'm not sure if thats enough.
Not sure if that applies to the long spine variation, but I know the generic heart urchins and sand dollars have this issue.