ok so i have a Nikon FM and the light meter is kinda acting weird so heres the problem

when i change the shutter speed the meter light moves up and down like it should but when i change the apriture opening it dosent effect the meter at all.

like when i move the shutter speed it will go from this
.+
o
-

to

+
.o
-
but if i change the apriture it dosent effect it so i dont know what is up with it