my guess is that you are using the camera's autofocus points and they are not focusing on the people, or if they are, perhaps just a titch behind the people, this is why i use manual focus almost 100% of the time, there are very few instances that i switch back to autofocus, now if i had a USM or equivalent lens with full time AF i would leave it there but sadly i dont
, anyways, if that is not the problem then perhaps your lens' are simply soft, or you are trying to use to large a DoF, at small apertures, this thing called diffraction limiting starts to come into play and robs your images of sharpness, each lens is different but normally around f/16ish i would say is when this becomes a problem.