Solution : https://service.sap.com/sap/support/notes/1051445 (SAP Service marketplace login required)
Key words : 
individual luws differ considerably, qrfc logical units, relevant support package, run time' parameter, increased maximum runtime, selected maximum runtime, outbound scheduler selects, individual luws, maximum runtime, maximum level
Related Notes : 
       
| 1324454 | qRFC scheduler is still set to WAITING status | 
| 1163359 | Load methods using SMQS or SAPI-controlled to transfer to BW | 
| 1118297 | XI runtime: QRFC processing of messages | 
| 1067854 | Queues remain in status READY in transaction SMQ2 | 
| 961154 | tRFC/qRFC LUW missing in connection to DB2 database | 
| 888279 | Regulating/distributing the workflow load |