are you tapping the key totally into the slot?
does it fit all the way in?
perhaps there is some grease or sumsuch blocking the key from fitting.
I have gotten stubborn keys to go in by slightly inclining them like a ramp so they don't hit the neck on installation.
be sure the keyway on the collar is clear of debris...

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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!