I have a 15mm single pipe system that was (until recently) working pretty well, but since we had an extra radiator added (downstairs), the downstairs circuit has not been circulating properly (if at all). The plumber who fitted the new rad says that this is due to the length of the run, and the number of undulations in the pipework which is creating airlocks (though it used to work before), and the pump (which he uprated) can't cope. He has suggested abandoning the existing circuit and creating 2 new (much shorter) drops from the upstairs circuit.
well its not the ideal system to have but they were used a lot in the early days if it worked okj before it does sound like he might have piped up the rad wrong dont forget the water does not pump around the rads it only goes on gravity and if he has put on a new pump it may be running to fast and therefore bypassing the rads
also the rads will need ballancing from the beginning of the circuit to the last on the run try turning the pump down first and try the flow from the boiler and see if it gets hot and then the return should get hot eventually
the pump can cavitate then there will be no circulation just bubbles
so turn it down as low as it will go