Make sure that the force you are applying is directed towards making the stem slide further into the arm. If you are just pushing the stem and the arm at the same rate then the tapered part of the stem can't get pushed into the arm far enough to grab on.

You might have to use a piece of wood to hold back one piece while pushing on the other.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech