Fault finding 101 !
Have you swapped them with known working speakers? have you swapped the amplifier with a known fully working one? have you checked the wires connected to the speakers are intact all the way back to the amp?.. etc etc.
Try these things, and see how you go. If you can't do them or all seems to be ok, then you could get a diagnostic check done at the BMW dealer or at a reputable car audio shop