Hello: It sounds like the lower thermostat is not in a good mood 😛 If it shows any signs of having been wet, or burning/carbon around it, than it clearly needs to be replaced. (you could post a photo here) Before this, simply exercise it a bit, turning up and down, to see if that frees things up a bit. Additionally, make sure it is tight to the side of the tank and that there is insulation covering it. I’d also leave it set a little higher than it is currently set for to see if you notice a difference.
Of course, it’s a bad idea to make sparks and electrocute yourself, so get help if you aren’t familiar with 240 volts 😉