What are you using for the release tool? Perhaps you need something slightly thicker and/or stiffer? My thought being that you may be prodding in the right spot and indeed contacting the trigger point, but whatever you're using is flexing out of the way or slightly too skinny to move the latching bit far enough?

I only bring it up because I had issues with the lock cylinder in my SDL for that very reason. The one installed when I got the car was an aftermarket and was *VERY* sloppy. So sloppy that I had difficulty getting the pin held in place. I wound up using an eject tool for a PC CD drive and wedging it slightly to get enough clearance.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]