There's really not much too them.
Are they inserted into the header correctly?
Are they facing the right way?
Is the sensor in the correct pocket?
Is the sensor inserted into the pocket?
Is the system used as a pre-heat system or is the system through a coil in the tank.
The pump generally only starts when the temperature in the solar manifold is 2 or 3 C above the temperature of the cylinder.
Could be a faulty sensor or the controller reading the incorrect temperature.
Is there anyway you can feel or test the temperature of the header to see if the readings are correct.
Whatever, there is an issue because 26C is extremely low for a temperature from the evacuated tube system