Hello,
Check whether any substitute is active for concerned user. You can verify the same at transaction SBWP : Settings -> Workflow settings -> Adopt view.
Refer to Table HRUS_D2 whether any substitutes are assigned to the User
If the results in (SWI1) and SWI5 is positive then there is a possibility that within the pool of agents of a workitem; one agent has RESERVED the workitem so he/she can see it but workitem wont appear in other Agents inbox. Try to un-reserve or replace the workitem via SWIA and then workitem will appear for all users again. Check results again in SBWP
Regards
Sandy