We're today running with OS390 Server V3.1.2.20 and a few months ago we had
a similar problem with caching of the primary disk storage pool.
The level of pool occupancy went higher and higher. I set the migration
threshold to 1% and it stayed high. Just to give it a chance I switched off
caching and it suddenly was possible to completely empty the pool.
I didn't take the risk of switching on caching again so I don't know if the
behaviour changed during the last ptfs.

Regards,
Michael