I've experienced this same thing with my Topeak Joe Blow Sport. I think it has to do with the valve sticking a little bit and that stickiness will make it quite difficult for the pump to overcome as the pressures are already quite high. So I always release a little bit of pressure to make sure I don't over-pressurize the valve or something because that'd be just my luck! Sure it's rather inconvenient to a small degree, but I'm glad I'm not the only one who has experienced this before.