According to the following sentece, do you know what is the exactly limitations of FO Sensor Model ?
"Both Sensors must be of the identical model type, and have the same signature set and software
loaded. (One of the two Sensors may be a "Failover (FO)" Sensor model, which is a fully functional
Sensor limited to operation as part of a failover pair; it cannot operate standalone."
McAfee Network Security Platform 8.3 IPS Administration Guide (Page 118)
An FO type sensor must always be paired with a standard type sensor in order to get updates. If you have a 'standalone' FO type sensor installed on NSM, it won't take updates (policy, sigset, etc).
Once the sensor is paired with a standard sensor, the manager is able to update both sensors.