I think part of your difficulty is the lens you are using with the 10D.( This is not a snide comment at the 100-400 IS L - It is one of my most used lenses, but it CAN be soft if not used carefully - even on a 1DMkll!) I used a 10D until I replaced it recently with a 20D. I also own a 1DMkll, so I am familiar with all three Canon bodies.
But the autofocus in the 10D will not perform at its best with an f5.6 maximum aperature, which is what the 100-400 f4.0-f5.6 IS L is when zoomed all the way out. A faster maximum aperature lens - f4.0 or f2.8 will help the 10D quite a bit, particularly if the light level is low.
This is a nice way of saying the 10D works better with nice prime L telephotos like the 300 f2.8 or the 400 f4.0 DO. I have many lovely shots with the 100-400, but the 10D definitely will focus better with these faster primes. Kind of makes sense - More light = better focusing!
The other think that will help the 10D, is to use ONLY the center focal point. I was never happy with the focusing of the 10D until I turned all the other focus points off. And you must shoot in the Creative Zone ( as Canon calls it ) not the Basic zone to do this.
The 10D can take lovely pictures with the 100-400, but the 20D and the 1DMkll are faster in dim light. But I have shot images in darkness that the 1DMkll could not autofocus in either - but that was with a tripod anda 24-70 L, not a telephoto.
SO the failure is not in the tools per se, but just expecting too much out of them !?