Your key is most likely worn down. Look at the points along the way of the key. If they're rounded then thats your problem more times than not. Take it to a locksmith and they will be able to visually look at the key, right the number value's down and make a new key with sharper points... Just ask them if they can duplicate off of a broken key. It they say yes, they can get you a new key that will work fine.
Car enthusiast for the car enthusiast. "I'm tired of being lucky." My Wife "I'm mad because I'm not stupid." My Wife
