loss of pressure & cold radiators at the bottom?

We have a Worcester Greenstar R40HE combi boiler.
The pressure is dropping on an almost daily basis though we cannot find any signs of leaks.
I've tried bleeding the radiators but have several where they are hot (+ water on bleeding) at the top but cold at the bottom.
We have been told we can force the air out by turning off all the radiators, then just turning on 1 at a time. If this is so should the system be running and is there any specific order.
The system was only fitted and commissioned last year.

many thanks.