3d printer thermistor does not work when attach the nozzle

Hello

i have one strange problem. When I take out the nozzle, the thermistor does work and it heats up to the point I order. However, when I attach the nozzle the temperature rises up to the 10 degree less than the desired temperature. For instance, when I give 250 degree it goes up to 240 degree and stops there!!

Does anybody have any idea?

Thanks