Heating gets colder after 30 minutes

Hello, unfortunately I haven’t found a thread about my problems yet. My V70 D5 MJ2009 works normally at cold temperatures. I have the automatic air conditioning at 21 degrees and after a few minutes comes really warm air – so far ok. If I am on the way now about half an hour, the air on the nozzles becomes cold – so that it becomes much colder in the car. Only when I turn the climate to e.g. 27 degrees then the exhausting air becomes warm again. This condition then does not change even during a long journey. This behavior is reproducible. I don’t know where to look without driving directly to the friendly one… Do you have an idea?