A keeper rate of ~30% with the 100-400L? Honestly, I'd send the lens to Canon. My experience with the lens across three bodies (7D, 5DII, 1D X), the keeper rates were >90%. The only time it struggled was with small birds in thickets (the 600 II locks onto them, but with the 2xIII it behaves like the 100-400 in those situations).