Mop Pad unscrews during mop cleaning

My machine is having a constant problem.  When the machine returns to the base during a cleaning session to clean the mop pad and refill the water, the left hand mop pad unscrews a bit during the cleaning process and I have to go and manually screw it back in and then continue the cleaning.  Then it cleans fine until the next pad cleaning.  Is this happening for some reason?  The other mop head has no issues.