IPSLA not working

hello,

We have the IPSLA module installed on Netflow 9.1 Professional. The module seems to be very buggy. We are trying to query 4 Cisco 2811 routers only one works..I have previously contacted support which they have called to provide support but after an hour of troubleshooting I was told the problem is with the routers. I have verified the configuration and even changed one of the routers hardware and the problem is still the same. If from the router command line I run
sh ip sla monitor statistics 101 I get the results therefore I don't assume there is a problem with the routers.
Is there anything else I can try,

thanks

Ramzi