Thermostatic Controls - Thermostat won't go on properly

We have a T498A1786 Honeywell thermostat. We heat electrically. Now, my problem is that the thermostat won't turn on until I turn the knob enough to hear a click. Then it will go on, however, in my sons' room it clicks at 25 degrees celsius and that's way too hot. I heats at 25 always, if I want to put it at 20C it goes off and doesn't go on until I turn the knob to 25C. In other rooms it clicks at 15 which is okay and in another one it clicks at 20C. How can I fix it so that the thermostats will go on to the selected temperature I need?
Thanks!

My best advise is get a line voltage t-stat that is digital control, Honeywell has a higher end t-stat that is programable otherwise non-programable t-stat.

Home Depot has (Down here) has Honeywell Programeble t-stat, and also has RiteTemp. Those are going to give a you a much better temp control.