Heat pump issues

I had an air source heat pump installed in the summer and up until now it’s worked pretty well. I can’t seem to get it heating the house well. When it’s working, the internal ambient temperature doesn’t go above 21°c however the heating only seems to last about 20 mins , gets the new radiators luke warm, and then the pump turns off. It’s frustrating because I have my 87yr old dad here and he’s shivering. This will be no good once winter really hits. I know nothing about the system, is there an outlet flow temperature above which it won’t work? I was given no instructions on how to operate it, and the instructions I downloaded for my unit are so complicated, they may as well be written in Chinese (although they are actually in English.) Can anyone help please?