Sounds like the valve sticking.
I have the same problem on one of my bikes.
Solution: Either depress the core momentarily, to allow a small amount of air out and free the valve, before attaching the pump.
Or attach the pump, give it a few strokes until resistance is felt, then tap the pump connector where it attaches to the valve - you should hear a small 'click' as the valve frees itself.
You should be able to carry on pumping normally after doing this.