if your key is original - it could be worn down to the point where it won't activate the tumblers. I had that problem on an old North American car. I found an innovative locksmith who cut a new key using a piece of match book cover under the key to raise up the key slightly in the jig before cutting. In my case, it cost me two keys as he had to use 2 pieces of match cover to get it up high enough to engage the tumblers. Before tearing out the ignition assy. it might be worth a try.