I had such a case, after 2 years of use combi,pressure began to rise then the drop.One day, boiler all stopped working.The reason for this is the expansion tank.
He is behind the boiler, in an accessible place.
Expansion tank serves as a pressure compensation liquid and air.Which gradually, is lost.And that is the cause of failure boiler.To boiler, earned in full force,we need to upload the air in the expansion tank.For this we need a pump with a pressure gauge.
First, drain the water from the combi.
And after, using a pump, pump air into the expansion tank.Air pressure should be between 0.6 - 1 bar.
And your boiler will work as before.