ferroli combi boiler Pressure?

I re-pressurised my expansion tank about 18 months ago using instructions below:-

1. Switch off the boiler. Ensure that there is no water pressure in the system.
2. On top of the expansion vessel (grey object with standard tyre valve on top, viewable from the top of the boiler) inflate with a bike or car pump to 1.5 bar (22 psi)
3. Charge your water system pressure to 1.0 to 1.5 bar.
4. Start your boiler

I've no problems, until this week, found i'm losing pressure again, I tried the above again but found the expansion vessel still around 1.2 bar... Any ideas? when I top the boiler up it runs at about 2.5 bar and empties when off, but does not seem to be coming out of the release pipe at the back as it did 18 months ago. Any ideas??