It makes a difference on my bike where the valve is. If it's down between 5 and 7 o'clock, I can't seem to get it to work. I'd have thought pointing down owuld be best... but 11 or 1:00 work best. Getting just the right angle seems to be the trick. (I have the same issues a little bit with the car tires... but my plan is to take them in tomorrow ...)