tshark not able to save already captured packets

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My application(SIPP) able to receive the packets, but when I check the save pcap file I am unable to see the packets, after call we close the tshark using cltrl+c, these all happen using automation script, but when we give a sleep of 2sec, I can see the packets on saved pcap file this happen most of the time but not all time. My question is if application able to receive the packets why tshark is not able to capture it and save it ?