Tone knob:
Many bass guitars have a tone knob that affects the overall brightness or darkness of the sound. If the tone knob is turned all the way down, it can reduce the volume of the instrument. Make sure the tone knob is turned up to full to ensure the maximum volume potential.
Broken Volume Pot
One possibility is that the volume potentiometer (pot) is damaged or broken. If the pot is not functioning properly, it may not be able to reduce the volume all the way down.
Output Jack
Another possibility is that there is a problem with the output jack. If the jack is not making a good connection with the cable, it can cause the volume to be reduced. Try wiggling the cable around in the jack to see if the volume changes. If it does, the jack may need to be repaired or replaced.
Grounding issue:
Inadequate grounding can also cause unexpected drops in volume. Check the wiring of your bass guitar to ensure that all components are properly grounded. Look for loose or disconnected wires and make sure the bridge and other metal parts of the instrument are properly grounded.
If you have checked all of these things and the volume still won't turn all the way down, you may need to take the bass guitar to a qualified repair technician.