CRM Asynchronous

CRM 2016 on premises

Bit of a strange one. 

I'm using a data importer to import records to a custom entity. On that entity I have a custom workflow which is set to run in background.

When I tried to import some 20,000 records, the workflow only fired for about 16,000 records. I deleted the records that didn't fire the workflow and re-imported them, only about 9,000. That's strange enough, but as I reduce the amount of records imported, the number of records firing the workflow decreases as well. ie just tried importing 10 records and only 4 fired the workflow. Re-imported the 6 and only 2 have fired the workflow. re-imported the 4 remaining and

I have even rebooted both the SQL Server and the CRM server … no change.

Any ideas what could be the cause, or where to start looking. The records that failed to fire the workflow, have not been altered , neither has the workflow, it is just a hit and miss to weather the workflow will fire or not

so far its taken 19 hours to try and import these records, surely that's not right !