nip down to your nearest hydroponic shop and get them to test it for you, this ballast has an internal ignitor that ignites the hps bulb so it could be faulty, if it is the bulb then you can buy a new one right away, if not, then claim a ballast under warranty or buy another.
metal halide bulbs have the ignitors built in and you can hear them clicking at lights on.
a metal halide ballast will not work with an hps bulb as there is no ignitor in the ballast or the bulb, but an hps ballast will work with a metal halide bulb as it does not matter if there are 2 ignitors, one in the ballast and one in the bulb.