I'm not sure where you live, but the common reason for this is key tumbler wear, or accumulated powder and grease from years of lubing the tumbler, gelling and generally being stiff in the cold.

I'd spray LIBERAL amounts of WD40 into the lock, until it comes out clear, then use LIBERAL amounts of brake spray cleaner... beware... the brake spray can strip the paint from your door, so protect it with tape, and plastic beforehand.
after the brake spray gets the tumbler turning, and the spray has ALL DRIED up... a TINY spritz of graphite powder should keep it lubed.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!