Not receiving Netflow Traffic IN

Hi,
I'm currently running Netflow Analyzer 4. I have set up Flow monitoring on one of our clients routers in order to analyse a specific interface causing problems. I have a number of the interfaces Managed including the Ethernet, and SNMP has been enabled and is working fine. Most the interfaces that I'm collecting are working fine, but one of them (the one I want to analyse) is only giving me the "Traffic OUT". I'm not receiving anything detailing the traffic IN on that interface. I know that there is traffic coming in, as I have network monitoring tools monitoring the bandwidth - which is about 90% utilized.
The other interfaces are fine, I'm receiving IN and OUT.

Why am I not receiving any FLOWs detailing the incoming traffic? And more importantly, what do I need to fix it?

Thanks