No NetFlow Data generated by Cisco 2610

I've got 2 Cisco 2610, same IOS, both not generating data. (I've also some 827 models which are generating NetFlow Data just OK, so I now everything should be working).

On the Ethernet0/0 interface I've got:

ip route-cache flow

In the general config I've got:

ip cef
ip flow-export source Ethernet0/0
ip flow-export version 5
ip flow-export destination 10.1.3.172 9996

Here's the output of show ip flow export:

Flow export v5 is enabled for main cache
Exporting flows to 10.1.3.172 (9996)
Exporting using source interface Ethernet0/0
Version 5 flow records
0 flows exported in 0 udp datagrams
0 flows failed due to lack of export packet
0 export packets were sent up to process level
0 export packets were dropped due to no fib
0 export packets were dropped due to adjacency issues
0 export packets were dropped due to fragmentation failures
0 export packets were dropped due to encapsulation fixup failures

show ip cache flow:

IP packet size distribution (0 total packets):
1-32 64 96 128 160 192 224 256 288 320 352 384 416 448 480
.000 .000 .000 .000 .000 .000 .000 .000 .000 .000 .000 .000 .000 .000 .000

512 544 576 1024 1536 2048 2560 3072 3584 4096 4608
.000 .000 .000 .000 .000 .000 .000 .000 .000 .000 .000

IP Flow Switching Cache, 278544 bytes
0 active, 4096 inactive, 0 added
0 ager polls, 0 flow alloc failures
Active flows timeout in 1 minutes
Inactive flows timeout in 15 seconds
last clearing of statistics 16:30:06
Protocol Total Flows Packets Bytes Packets Active(Sec) Idle(Sec)
-------- Flows /Sec /Flow /Pkt /Sec /Flow /Flow

SrcIf SrcIPaddress DstIf DstIPaddress Pr SrcP DstP Pkts