you may have bought the correct physical replacement but did not have the correct electrical
specs. volume controls are usually log devices where halfway point on rotation is not half
the total resistance.
look up volume, pots, tapers, etc. Verify with your service manual for your unit. it should say
something like audio taper.
otherwise as the above posters indicate it may be a soldering issue (maybe since reading
in between the lines, the control do vary the volume - just not correctly and these are
three legged fixed and you can't wire them out of order or backwards)
if you pull the new unit, using a VOM/DMM measure and compare the two. use any
two of the (two) pot's legs and consistently measure. have both face you in the same
front OR back orientation, pick the two left OR the two right. find halfway on both and
measure and compare. should be in close range.
this will tell you whether you have the correct replacement. or simpler to read the
description for the pot you bought unless you pulled it from another unit.
and even if you did pull it, there should be mfg markings (maybe) for component
value and/or product number - this last item allows you to check online.