The hammer action of an SDS drill has to interact with the mass of the bit or chisel. Essentially, the bit needs to bounce back from the impact with the work so that it arrives at the percussive device at the right time. Sometimes there's a nice resonance or near-resonance, but with very long drill bits or heavy chisels you might feel that there's a sort of dullness about the feel of it. All you can do is not push, so the bit has the chance to slide back fully before being struck.
Bill