Yaesu ft 840 amateur radio very quiet micraphone?

i have a yeasu 840 and the mic audio is very quiet and tinny , it is a astatic teardrop power mic, i have noticed that when i turn up the mic gain on the rig nothing happens, it done this with the original yaesu mic aswell , is there any adjustments i can make to bring the audio up abit? its a brand new battery in the mic