Why cant I move my apps from main storage onto a sd card

When for instant i download an app it will say low storage memory even tho i have a sd card inserted but when i go into apps and try to move it onto my sd card it wont allow it there are no option for moving onto sd card only internal storage and main storage is displayed and im sick off it i have tried moving the app files onto the sd card via a computer but then when i insert it back into my device and try to run the app it will say app not installed please help thanks.