FX Loop Volume Control
Your Volume control needs to be cleaned. Use a control or contact cleaner with lubricant only.
Do NOT use head cleaner without lubricant, this will make it worse.
Do NOT use WD40, nor TV tuner cleaner such as Blue Foam.
If this does not solve the problem then check to see if the pot is loose. If so it may need resoldering.
If the pot is jammed and does not turn freely then it is damaged and needs replacement.