the most usual cause of a similar problems that we have seen was in MS or third party firewall in place which blocked the packets (wireshark operates on lower level then software firewall so it is still able to capture incoming packets).
Try to disable any possible FW, or temporarily disable netflow on the non-problematic Nexus router, open performance monitor (perfmon.exe) on collecting box and add all Solarwinds NetFlow counters and if you cannot see anything here, it means that collecting service is not receiving any packets (again most probably because of some another SW blocking the packets).
Thanks for your response. Unfortunately, there is something else causing this issue. I've opened a support case with Solarwinds.