Problems with plastic central heating pipe underground

We had a modest extension built a few years ago, but now no longer get any heat from either of two radiators installed. I've checked the pipework - there are two tees off the underfloor 22mm flow and return to 15mm copper, which then connects to grey plastic with push fit(?) connectors. The pipes disappear from the house through an outer wall underground, and eventually must appear somewhere within the concrete floor of the extension. Hot and cold water to supply the utility room in the extension are connected in much the same way. These both are fine. I've just drained and flushed the main heating system, and while drained tried to flush through the extension pipework. I dissembled the copper pipes from the two compression tees, and connected our garden hose to one, with a pipe into the kitchen sink from the other. With mains pressure applied, there was only a tiny trickle out of the other pipe - even when both radiator valves were fully open. Has anyone any idea what has happenned? I would have thought the plastic pipe underground would have been wrapped in insulation, then encased in concrete - but it seems as though one or other of the two pipes has become constricted in some way. How do the copper to plastic connectors work? The seem as though they are push fit.