My 1998 Furby has Sleepy Bitch Disease AKA MSA/"Me Sleep Again" and the usual method of fixing that (you're supposed to flip them upside down and shake + tongue depress/reset) isn't curing it.
Based on that, I think it's a hardware problem with either the tilt sensor or light sensor.
I'm hoping it's the one tilt sensor that I can actually access, albeit with difficulty, to clean because if it's the light sensor there's nothing I can do. The light sensors are pretty proprietary because of the shape and aren't used in basically anything anymore. The only replacement for those comes from other Furbies and I'm not looking to cannibalize any Furbies. If I ever obtain more than the two I have it definitely won't be to destroy any of them.