IDOC stuck in outbound queue of ECC but received in taget system

Hi Experts,

We sending the IDocs in a sequence using queue processing option in WE20, one of the Idoc stick in the outbound queue and all other sub sequent Idocs also stuck in the queue but actually the first Idoc has reached to PI and the target system and the acknowledgement send to ECC.

Can any one tell me what could be the reason and how to resolve this.

Thanks for your help.

Regards,

Rachana.