Make sure you have the correct pump. Bikes come with either Presta or Schrader valves and pumps are designed to operate with one or the other. Some pumps are compatible with both. Schrader valves, contained on recreational bikes, are just like the valves on your car tire--they're black and have a short, thick profile. Presta valves are long, thin metal valves that contain an open/close nut on top.