what to do!

so I have a small problem, or rather, my brother has it. My brother also collects furby and already on February 12 he will receive his first boom, well, he has several furby 2012, one of them is the color of twilight. Well, in short, after the next purchase of batteries for his furby, he inserted them in Dave (his name is Furby), he started to get very buggy. After that, he came to visit me and told me everything I have listed, we put new batteries in his Furby, but he is still buggy !! there may be a problem in the motor, but so far this is one of the versions why it doesn’t work well, but it’s not yet a fact that it’s true. No matter what batteries we put in (we took new ones), it lagged. I hope someone knows what’s wrong