Non-durable deletes remove a record from the index immediately and the space within the wblock on disk is available to be asynchronously reclaimed by defrag. For a wblock to be eligible for defrag, the occupied space within the wblock needs to drop below the defrag-lwm-pct.
I would need to see the associated warning in the logs to know if the problem is lack of free index space in RAM or lack of available wblocks in storage. Meanwhile here is a KB on the defrag system.