IP traffic flow not "capturing" all traffic

Hi,

I just installed ntop and setup the ip traffic flow on mikrotik. ntop is receiving the flows, but the avg traffic and network load showing is only about 1/4 of the real traffic.

for an example the current traffic displayed on ntop is about 30mbps but the real traffic is 130mbps.

is there any special configuration in the way mikrotik send the flows to ntop ( like cache entry time or something..)

ps: this mikrotik is a transparent traffic shaper (bridge with contrack and ip firewall enabled) connected to our upstream provider and core network.

thanks