Six Sigma Project – Help Required

For the cheque rejects in clearing, Clearing Deptt. sends Referral mails to the concerned branches asking them to revert before a specified cut-off time, whether they want to pass/reject the cheques pertaining to their customers. But most of the time, Clearing Deptt.do not receive replies from Branches within the cut-off time. And this results additional effort and cost in the form of follow up mails and telephone calls. Can somebody help me to do a six sigma project to improve the TAT of this process??