Hi,
It is a common occurrence on the ignition barrel.
Unfortunately , the chances are it is only going to get worse.
There is a great write up on how to remove the ignition barrel and check the brass gates the the key works on. May be on here or search theinterweb.
Basically it is either, remove some gates, change the barrel, or you can clean the barrel up and hope it does not happen again.
Not a difficult job to do.
Cheers