I'd guess it could be either a worn/sticky lock cylinder, or more likely a worn key. I believe the sole purpose of the uppermost notch on the key is to hold it in the lock when in the on position; if that is worn you will be able to pull it out.

If you've got an unused (or infrequently used) newer spare key, try that and see if it still does it? If it does then it maybe the cylinder that's wearing out, or a tumbler is sticking or stuck in its shell. Try a dry lubricant such as graphite powder available from a locksmiths (best not to use WD40, oil etc as they all get tacky after a while and can cause tumblers to stick.)

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