I am not sure I can completely solve your issue but I can help you troubleshoot it.
You did not state how old the trouble child crockpot is so here are my suggestions.....
1. If it has any age at all or you use it frequently you may have stripped the inside of the knob. If it isn't making solid contact and you are turning.... you may be turning air.... thus not actually turning the crockpot up as high as you think you are. If it is an electronic control panel check into a calibration. Maybe somewhere down the road it has been jarred or something. I have never heard of a calibration being done but it might be worth a shot if it is not to old of a model. Just look/talk around.
2. The thermostat is bad or dirty. I am not sure where it is in your model because I haven't looked it up but that may be worth looking into. If the "stat" is not receiving the correct temp feedback it won't put out the correct heat to control your food temp.
I hope you find a solution... check these out and if you have any questions feel free to email me [email protected]