We have noticed some threads on packet capture and flow collection issues/questions related to NSX. Take a look at this option from ReSTNSX - Cloud Check.
Cloud Check is a tool that allows you to centrally capture flow data from NSX or vRNI for analysis. This has been very helpful to customers looking to analyze VM traffic in preparation to move to the cloud or simply for troubleshooting reasons. The data is collected real-time (dFW or vRNI) or historical (vRNI) and presented in simple graphs and tabular data.
With the dFW option, only a prepped host for NSX Firewall is needed - no external collectors or analyzers. Additionally, the dFW approach observes all flows, not sampled like IPFIX/Netflow.
Check out this demo video -