In my experience this is caused by one or more of: gear, flaps, ordinance, fuel load, trim, or throttle calibration.
I once led a jabo strike in an AW scenario. We flew for 30 minutes to the target, and I seemed to be always having to slow down for my flight to stay with me. We got to the target and dove in for the strike. I lined up and pickled off my bombs -- except there weren't any! I had forgotten to load them.
Reported "damn...two misses" to the flight.

popeye