Work item XXXXXXXX locked by user XXXXXXX (enqueue error)

I have an issue with Appropriation request workflow process.

There are 4 approval levels. Agent indentification and approval happens in a loop.

Occasionally, the workitem fails with the message 'Work item XXXXXXXX locked by user XXXXXXX (enqueue error)' The workitem is  getting locked by the previous approver. Is there any specific reason for this lock or do I need to add a wait step for each loop?

The approver, approves the workitem from his Business Workplace. I checked other threads, but I am not able to figure out a reason. These are all synchronous processes.

'Index access to agent tables' is a background step?

Thanks,

Surya