This is most likely an issue with MTU. SteelHead/SteelFusion used to utilize 5 template fields for IPV4 Traffic but with the support for IPV6 template added, now there are 10 template fields, increasing the frame size. The DF bit is set for the flow Packets. To be able to accommodate this, MTU on the Netflow Source appliance (SteelHead/SteelFusion) should be set in such a way that any device on the path to Flow Gateway doesn't need to require Fragmentation of the packet (which will cause the packet to get dropped as DF bit is set). In other words, SteelHead/SteelFusion appliance MTU should be set to the lowest supported MTU on the path.
In order to determine the min MTU supported on the path to FG, please use steps mentioned in the below KB
https://supportkb.riverbed.com/support/index?page=content&id=S15913