Help On Landing Multi Engine aircraft

S~! I have been having problems landing. Say I fly a Gulfstream to a concorde the same problem occures on any mult engine aircraft, this happens sometime around decent and apprach. I turn off AP, gear dn , flaps , brakes all as needed. I ajust throttle but when I look at the engine panel only 1 engine is selected out of 2 or more. I end up having to try and land with one engine at full throttle or wher ever it happend to be at time of occurance. VERY VERY frustrating. HELP Please 
