I'm sure if you play it long enough your body'll figure out how to do it.

Try this exercise that Gary Chaffee suggests in Technique Patterns. Hold the stick in between the index and thumb (palms down, or at least almost down if you're doing "American") and then practice using each subsequent finger to play strokes - one at a time. For instance: four 16ths with the middle finger, four 16ths with the 3rd finger, etc... Do any series of note-values and fingers you can think of. Make sure the other fingers (except the index and thumb) are not touching the stick. The reason you may be having trouble is that the finger motion in the German and/or American grips is more like pulling "in" toward the palm and less like the upward motion of the fingers in French grip.