WaveyGravey, it sound like you need to 'burp' your presta valve. The tiny rubber seal inside the valve stem often gets 'seated,' gets sort of stuck in place. Before you put the chuck on, press the valve in and let some air out of the tire.

Oh...and remember to unscrew the little brass nut on the valve first, too. These are the two most common issues that cause the problem you describe. One or the other of these happen to me about once a week. I say, ' what the heck...? Oh, I forgot to...'