If the bulb is good and it gets presented with 12V then it will come on - if a bulb is not coming on then it's not seeing 12v.
I suffered the same sort of problems for three years with shoddy lamp holders and poor connections and became a martyr to my 12V halogens in the kitchen.
After replacing them with 8W mains neons (worktop under cabinet) I have my life back again.
Peccavi
January 2011