Samsung AQV12NSBN not heating enough

[Arkebuse]

Hello everybody ...

I am calling for your help because my inside unit doesn't give the warm air it gave a few days ago.
This configuration, one outside unit and one inside split have been installed 2 years ago without any big problems till now.
I only start those units when it starts to be cold inside, so the split works only in warm mode.
My problem is that the split unit gives warm air after the startup but never reach more than 18°C even I select 27°C using the remote control. The airflow is just lukewarm.

Any idea for first checks ?

Thanks everybody :)

[Brian_UK]

What is the ambient temperature and what is the design temperature for that unit?

Have you had any work done on the unit since installation?

[install monkey]

any lights flashing on the indoor?

[Arkebuse]

A temperature of 8°C mesured outside, 24°C selected on the remote control and only 18°C inside...
It seems to me that before this problem, the airflow was warmer when I started the flip in the morning.
Everything seems to work fine : no light blinking, no error.
What about the maintenance to do on these units ?
The filter in the flip is clean. Is there anything to do ouside ?
Thanks for your reply;)
Stéphane