Problem with the pressure in combi boiler?

My valent combi boiler has problem with the pressure which keeps on falling which causes the radiator not to heat up. I have checked all of the radiators there is no air in any of them, please help me by telling me how to raise the pressure.
Mithla Sharma, September 2018


done
Internal filling loop
https://youtu.be/XDZXGm_KY9U

External filling loop
https://youtu.be/WEOKp3MWzJk

Anon, December 2018