Capturing FTP data on wireless
 

I am working on 802.11b on Dlink 520+cards on linux platform
I want to capture ftp packets using tcpdump/tethereal. I start an ftp session with other machine. Initially, packets are captured by both of them but as soon as the ftp transfer is complete, other machine shows "File send", but tcpdump/tethereal by that time has not captured all the packets, and stops capturing at that instant only and does not go on further to receive remaining data and FIN packets.

Why does this happen and how to resolve this??