Missing traffic on interface

I have configured netflow export according to the documentation but I have one router which refuses to provide netflow accounting on the serial interface. I receive stats from E0/0 and E0/1 but not s0/0. Below in the config on the serial

interface Serial0/0
description Frame-Relay
bandwidth 1544
backup interface BRI0/0
backup load 70 50
ip address xxx.xxx.xxx.xxx xxx.xxx.xxx.xxx
ip access-group xxx in
encapsulation frame-relay
ip route-cache flow
no ip mroute-cache
ntp broadcast client
fair-queue 64 64 0
frame-relay map ip xxx.xxx.xxx.xxx xx broadcast payload-compression packet-by-packet
frame-relay lmi-type ansi

Any idea's? This router is also behind a PIX.