How can I fix my Vokera Linea Max boiler which wont fire in am ?

I have a Vokera linea Max boiler 10 years and get it serviced yearly. However I had to remove a radiator last week and de sludge it and since refitting it the boiler won't fire if left off overnight, even though the pressure is above 1 bar. I have to increase the heating temp. and reset then it fires. When it fires it goes to about 2 Bar and works great.