I have 4 Rad upstairs and 4 Downstairs, I had a new gloworm system fitted 3 months back by a corgi engr. In the last few weeks, when I turn on the central heating via the dialer, sometimes it comes on, other times it does not. The recent problem is that all the rads upstairs get hot and two down stairs. The Rads have all been changed to new ones and bleeded. Cam it be the Pump, the valves or sludge.