Might be the relay contacts aren't making good contact in the transfer switch. With the rig unplugged and generator off, try cleaning them with some emery cloth (or fingernail file). Also, if you're a 50 amp rig, make sure your power source has power on both legs.
It could be low voltage too, it takes 110 volts to hold the transfer contacts closed when on shore power. Check it and see if OK.
'11 GMC Acadia SLT AWD
'11 GMC Sierra 1500 4x4 Extended Cab