The system is 12 volt but are the bulbs? If the bulbs are rated for 12 volts and the transformer is 12 volts you could have just got a hold of some bad bulbs, it happens. Re-check the voltage on the transformer and the rating on the bulbs. Get a test meter and with the bulbs hooked up check the voltage. If you unhook the bulbs and check voltage you may get a higher than nominal reading.
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