I had the same problem with one of my 2810 switches. We had purchased and installed two units at the same time on the same network. One was working with PCM, one did not.
To enable SFlow on the none-functioning unit, I had to set the snmp-server community **** to manager with unrestricted Write access.
Then I had to delete the device from PCM and rediscover it.
After rediscovery, PCM was monitoring the device. You can verify the SFlow status on the switch by typing "show sflow all" in configure mode. You should see sflow enabled and the destination address of your PCM host. If sflow is disabled, it means PCM cannot configure it due to a snmp community permission or name mismatch used in the discovery.