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.
Download the new ePolicy Orchestrator (ePO) Support Center Extension which simplifies ePO management and provides support resources directly in the console. Learn more about ePO Support Center