The heating does not work

Good morning,

I beg pardon my English, I use the translator, I am Spanish and I have a Opel Insignia to gas oil, and the heating does not work correctly, it has air conditioning and double zone, it only heats up to 75 ° F, the cold works well, it is only the heating and in both zones, the air sends it correctly both to the feet and to the other places.
What can happen to the heating?
It has me worried, I tried to reset the actuators but it did not work.