My upstairs and 2 downstairs radiators are not getting hot. Why?

We have a single pipe CH system with a gas back boiler. The radiator in the kitchen is cold, the hall is lukewarm or cold, 2 upstairs bedrooms warm and 2 other bedrooms cold. Had a powerflush (£350.00) last week. No improvement at all. Radiators are balanced,open and have been bled. Pump and boiler checked.Even when hot radiators are turned off the cold radiators remain cold. Any advise very welcome.
Thanks