Mitsubishi City Multi

[Zoran_Popovski]

I did it and even I changed refrigerant with new quantity without any result.

[marc5180]

20bar on heating is too low, something is stopping the system from ramping up. Check the discharge temperature and discharge pressure (transducer). Connect your gauges to the system and confirm the readings are the same.

[Zoran_Popovski]

When the sistem start discharge pressure is between 20-22 bars,but after 10 minutes pressure will go down to 15 bars and will stay there.Is there any possibility of filter clogged or compresor become weaker?

[marc5180]

When the sistem start discharge pressure is between 20-22 bars,but after 10 minutes pressure will go down to 15 bars and will stay there.Is there any possibility of filter clogged or compresor become weaker?

There are several things that will prevent the compressor from ramping up to achieve target condensing temperature/pressure - one of these is the discharge temperature.
Have you checked this? This can be done by the dip switches on the outdoor PCB.
As I've posted above - also confirm the pressure transducers are reading the correct value. It could be that the system thinks that it's running at 28bar (for example) because the discharge transducer is readibg 6-7bar out.
If you come back with this answer then we can work through the problem with you.
This needs to be checked before worrying about clogged filters in the system.