Wait!!!!! Let's look at this for a second....I know potty training is a real difficult time for everyone--or certainly can be. Kudos to you for being patient!
It seems to me that your daughter has a fear of losing control of her bowels and not being a good girl. (VERY common!!!) She wants to do a good job and just is having trouble with the on/off switch so to speak. I had trouble urinating when I was in labor. My body just couldn't remember how to coordinate all the muscles right then...
Giving her a laxative may make her lose control---her worst fear realized!!!! And do more harm than cleaning up a poopy mess for a few more days/weeks/months. (I know....sigh.....)
Letting go is one of the biggest struggles people have emotionally. I just watched my friend try to die for a long time.....help your daughter learn to let go now. It is a valuable life lesson. She needs to learn how to control the process. Just think how much easier her love life will be!!!!
I do not want to go against the advise of a physician but remember, Dr. Spock recanted after his methods were used almost biblically for decades. No one knows these things for sure. You can help your child better than anyone else. She may actually know what she needs herself. Try asking her. My kid worked out great out of the box solutions for himself that worked because he knew himself better than anyone. Try silly stuff too if she suggests it....(I don't know....singing about it to a pretend purple kangaroo while she's on the pot,or pretending she's on a ship....what does she see, how does the ship gently rock with the waves hitting it etc....)might just lighten the situation up enough so she relaxes enough to poop spontaneously....stranger things have happened.....
Good luck,my friend.
Thanks for being a good and loving mommy!!!!