Messages getting stuck in queue

Hello experts,

Out of 3 messages that come to XI for processing, 2 get stuck in queues. The status of these queues is system failure and error message is: "Function module "ENQUEUE_E_SXMSPFADDRESS" not found". However, on unlocking the queues manually, messages get processed. What can be the problem with message processing that they keep getting stuck in queues ?

Thanks,

Monika