It sounds like a calibration issue. I don't know about the 12FGA specifically, but on the 12Z, there is an hidden menu to configure the tx including calibration.

See: http://www.helifreak.com/showthread.php?t=20745

Call Futaba support line (see http://www.futaba-rc.com/techsupport/index.html). Don't email as they don't respond.

Alternatively, the official futaba support forum RCU is very good

Arnaud