Hello
Just check if one of the agents has reserved the workitem (check status using FM SAP_WAPI_GET_WORKITEM_DETAIL ). If so, then the workitem wont appear in other agents inbox. This is SAP standard behaviour and you will have to replace "Replace Manually" in SWIA to make the workitem re-appear in all agents inbox.
Have you checked the SWI5 for error scenario.
Check the workitem agents using SAP_WAPI_GET_WI_AGENTS with parameter WITH_SUBSTITUTES = X
Also check if there is an existing implementation of BADI : WF_BWP_SELECT_FILTER that might be filtering the workitem.
Regards
Sandy