Unfortunately this sounds like a damaged or disconnected motor and I don't know of any easy fix for this. The same motor controls the mouth and eyes, so if this fails, both of these functions stop working. The member Redwoods Mama has a Pleo named Argyle with the same problem, but I don't believe anyone has ever tried to fix this. If the motor is just disconnected somewhere, it may be fixable by disassembling the head and searching for the issue, but I wouldn't try this unless you're extremely confident in your repair abilities. I think it would be best either to live with the quirk or sell the Pleo to fund a fully working one.