The only thing I can think of is A) you haven't got the probe droids around the perimeter B) missed the speeders? C) Let some TIEs get away D) missed the odd turret (watch radar for red dots E) have been blowing the barracks before the stormies come out to get the shuttle F) missing the stormies? G) Missed some AT-STs? H) maybe...not killing those tall black oil rig-looking things near and around the shield generator?
I can't think of anything else...