When I got my first mmx silo launcher and rockets, I bought a 9V battery along with it - from Toys R Us - and it was an off brand. Needless to say, I had the exact same luck as you did.
Actually, the new set specifically says to use a fresh Duracell or Energizer Alkaline 9 Volt - and only those two brands. Apparently, most all other brands (Ray o vac, etc) don't have enough juice to fire off the igniter. From what I understand, that igniter is like dead-shorting the terminals of the 9V battery - that that will kill a 9V faster than anything (It generates 9V, but the amperage is so low it is practically non-existent.
I rigged the launcher to work with my Big Foot pad - the four D-Cells fire off those mmx igniters without a single problem. One suggestion I read was to rig a four cell battery pack or even a 12V gel battery to a 9V battery clip - you can then plug it into the battery clip in your laund controller and fire them off easily.
So, bottom line - the 9V battery is most likely the culprit.
Greg