Cacti Flowview - no data in files

[tollair]

Hi all, I have configured flow viewer for cacti 8.8b, I think I have it configured correctly.
I can see the files getting generated in my flow directory.

-rw-r--r-- 1 root root 92 Nov 20 11:58 ft-v05.2013-11-20.115721+0800
-rw-r--r-- 1 root root 92 Nov 20 11:59 ft-v05.2013-11-20.115801+0800
-rw-r--r-- 1 root root 92 Nov 20 12:00 ft-v05.2013-11-20.115901+0800

But they are all empty. All showing 92 bytes.

[root@PTC-SYSLOG-MGMT 2013-11-20]# flow-print < ft-v05.2013-11-20.115721+0800
srcIP dstIP prot srcPort dstPort octets packets

I have done a tcpdump and I can see the packets coming through from my cisco switch.

Any ideas here?

Thanks