I don't know of any striker adjustment procedure beyond looking, moving and trying. It can try your patience sometimes. For the OP having to slam it, the plate is too far in toward the center of the car. You might start by loosening the screws and moving the plate directly outward a small amount and then TIGHTEN the screws before trying it. Tighten them after every adjustment before you try it, so that it won't move in case you have found that sweet spot.

Some patience and you can get it right.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

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