Forgotten to soften the butter?

Grate butter coarsely, spread onto baking paper and stand for 10 minutes before using. Alternatively chop into 2cm cubes, microwave on 30% power in 10 second bursts until just softened. 

Avoid a "floury" white sauce

At the beginning of a white sauce always cook off the butter and flour over low heat for 1 minute to ensure the finished sauce doesn’t have a “floury” flavour. Remove from the heat before whisking in milk and returning back to the heat to thicken.