30% Keeper rate with 5D2 Center point AI Servo? Please help me : )

I know the 5D2 is not a sports or action geared body, but I still find my keeper rate dismally low when trying to follow my kids around. I have used several outdoor telephoto lenses in the past years (70-300mm USM, Sigma 70-200mm HSM, even the 85mm f/1.8) and while some of these lenses are not state of the art in AF speed, i still would expect a bit better keeper rate.

Any comment / ideas?

I shoot wide open sometimes, but stop down other to try and improve my success rate. Shoot at 2.8 at 200mm with the Sigma was very challenging (yes, I AF microadjusted it)...

I have tried different aiming points (face, eye, body, etc)

Maybe this is all I can expect for kids moving around and live with it and/or move to the Nikon D300/ Nikon D700/ Canon 7D, etc. in the same price rate as the 5d2 (Sorry I cannot afford the 5DIII or D800 right now)