Work item agent is locked

I have a SC that's been approved by a manager via the offline method.

We schedule report RBBP_OFFLINE_EVAL to run every few minutes to process these offline approvals and create a spool list. In this spool list, I see one entry that says:

Work Item 000001234567

Work item agent SMTIH is locked. E-mail was not deleted. Work item was not executed.

Does anyone know what causes this lock? I check SM12 and I don't see anything there. How can I unlock this work item?

Please advise,

SN