Verify that the IP address used to manage the device in NPM is the same as the address that's exporting NetFlow. You can change the latter with "ip flow-export source-interface" if you're using standard NetFlow v5 or v9.
We have something similar happening. Except SW stops receiving data from the device for netflow. To fix we had to change the device to point to our secondary poller, then back to the original poller. That fixed it. (this happened on 20+ different devices with a variety of hardware)
I submitted a ticket to see why we have to do this. No responses yet.