Today I tried to create my first custom rig - a hand rig. However, my rig has a problem and I can't figure out the reason why. The controller for the THUMB works just fine, while the controllers for ALL OTHER FINGERS are not controlled by the finger tip, but by the finger joint:

I can't wrap my head around why this happens, as I built all the finger structures in the very same way:

The only obvious difference is that the thumb (Daumen) structure consists of only two elements, while the others consist of three, but I do not think this should be a problem.
Can anybody tell me what I am doing wrong and how I can fix it?
Thank you!