I don't know what kind of valves those tubes on that bike have, but if they have Presta (the kind you unscrew) they can leak down a couple pounds a day, so you can lose 10 psi per week (sometimes more). ALWAYS pump them up to the correct pressure before riding. Even at ½ or ¾ of the recommended pressure, you probably won't be able to squeeze them so that's not a good indicator.