I was also facing the same issue with POWL inbox ( IBO_WDA_INBOX )toolbar few buttons are disabled and few are Enabled.
Disable buttons: NOT RESPONSIBLE, COMPLETE, END RESUBMISSION, CANSEL and ASSIGNMENT.
Enable buttons: RESUBMIT FORWARD ASSIGN TO ME and SUBSTITUTION.
I tried putting the below roles in SU01 to Manager but the Disabled button did not turn out in Enabled.
SAP_BC_BMT_WFM_NWBC_END_USER
SAP_BC_BMT_WFM_GP_SERVICE_USER
SAP_BC_BMT_WFM_INB_API_END_USR
SAP_BC_BMT_WFM_INB_API_SRV_USR
SAP_BC_BMT_WFM_PROCESS
SAP_BC_BMT_WFM_SERV_USER
SAP_BC_BMT_WFM_SERV_USER_PLV01
.
I think I need to go for WDC enhancement.








