Looks like you already have a case open on this - but offhand: It really depends. I would assume you have the vlan created and allowed on the trunk/po on each FI and that "show int trunk" on the FI from nxos shows the vlan is up and active. As well, whatever connects to the B side, check the upstream switch port that it connects to for the vlan, and whether its up (on the trunk/po, and spanning tree state). Those are the basics assuming you don't have something else in the config setup incorrectly, or windows is bridging the nics causing spanning tree to clip a link on B.
Unpinned would imply that there is no path out of FI-A. This can happen at any one of many points, i.e. IOM-A in the chassis has a port down, the FI is missing correct configuration to pass the vlan, the configuration is present on the FI (vlan and association to an upstream device) - however, the upstream device is missing the vlan/blocked the vlan.
I would login to the FI over SSH and look at "show interface brief" you should see the server listed in the output and the "virtual interface" that is unpinned. Issue "show interface vif XXX" and look at the reason it thinks it is down. As well, show vlan, show int trunk to verify that the vlan is assigned and operational on at least 1 network link.