OTRS - Sometimes mail do not fetch from POP or IMAP into the OTRS

[sunny]

Actually we are using GMAIL POP3 and found so many times mails do not fetch from POP into the OTRS so we checked Gmail and found mails are there and still that mails are not received into OTRS and after that mail if sender sends the mail again then we receive the mail into OTRS but still previous mail is missing in OTRS, so I have checked the logs into OTRS and no logs at that particular point of time or from that user then I checked header in gmail of that mail which is received in OTRS and which is not received in OTRS and no difference found in header of mail.

Any idea what else need to check or why it is happening?

Thanks in Advance.