If there’s air trapped inside of your radiators, you may need to bleed it. Trapped air will cause your radiator to have cold spots, reducing the efficiency of it. You can bleed your radiators yourself, and it can vastly improve the efficiency of your entire heating system.
If none of your radiators are heating up properly, it could be a result of a more serious problem with your central heating system or boiler. But, there are a number of smaller faults that should be ruled out first.