Probably caused by a build up of dust dirt etc, Could try some IPA (Isopropylalcohol) available from chemists, a few drops at a time, into the hinges. WHEN unit is unplugged battery out, and hinges are down over. Gently opening closing top while dropping them in, An old eye dropper, an old syringe, anything like that, That should flush any bits out, without taking the unit apart to clean, which is the other option.
SD, May 2012