Captain obvious I know, but are the valves open both ends?
Can of worms.
Valves at both ends are different valves.
There's one for you to control, then the other is the lock sheild. The lock sheild valve, it isn't always the case that it should be fully open, if it is, then it can cock your heating up, as that's for balancing the flow rate of the water, as the water will take the shortest path, leaving other radiators not as hot, or even cold. So the lock sheild should be set to regulate the flow, so that every radiator get's hot water without missing others out if it has an easy path to flow back to the boiler, missing others out.