Just an observation, from looking at your pictures, with no attachment and you close the levers and the pins go down. They seem to be working as they should, and I don't see how you can adjust pin depth, You can perhaps adjust spring tension, but in the picture, I see the pin bottom out on the bracket and stop at the 1/4 bolt and nut. If this is not true, then what is the purpose of the spring. If you can adjust the length of the shaft and pin, then you should be able to jam the lever down with as much force as you can. I believe the spring is there to set and hold tension.
Those pins probably are about grade 8 in hardness and strength, and you could make some more pins that are longer, steeper slope or whatever.
Can you take a picture of the adjustment area?