You can calibrate the thermostats, can't remember how but google would tell you. :-). They can be out.
Hope the rest end up okay! Must be awful for the poor kids :-(
Temps are throwing me since we have Celsius over here but if they all, including barely able to move ones are heading to the cooler side could you have the brooder turned up too high for two week old chicks?
Have you calibrated the thermostat to check its not hotter than its reading?