Why are you noting soreness as your main indicator of how hard they were hit? If something isnt neccessarily really sore it doesnt mean it hasnt been worked adequately or sufficiently.
Anytime there is scapular abduction, the serratus is being worked. My guess is you experience it the most with push ups because at the top of the motion you continue in abduction of the scapula after the actual push up has been finished and thats what hits them.
Jason 2549's post explains it well. Ive also found that when I do heavy DB Rows with a little english, the side of the body that is stabilizing me on the bench as the other arm is doing the Row, involves a lot of oblique and serratus recruitment.