Boiler pressure issue (Worcester Greenstar Junior Cond Combi)?

Last night, I noticed 2 of my upstairs CH radiators were cool at the top, so I decided to 'bleed' air from them.
Prior to this I had also noticed last week that the boiler pressure had fallen, but was still within the acceptable 'green zone'.
With boiler switched off, I bled the first radiator (bathroom, and also the smallest) until water appeared. OK so far. I then bled the largest radiator in the main bedroom, until all the air was expelled, but there was no sign of water at the bleed nipple.
I figured that I had lost pressure in the system, and would need to 'top up', so I started the boiler and opened the valve at the boiler to introduce more water, and this brought the pressure up to the black mark.
I then had to deal with a leak at the bedroom radiator bleed nipple, and had to switch the boiler off again.
I used p.t.f.e. to seal the leak, and I then tried to follow the same procedure, but ended up having to abandon the job at midnight, (bedtime) with no pressure and air in the system.
Apart from that, it's been a great boiler since I bought it new, 13 months ago. Can anyone please advise?