DD peeing in potty great...but no poo, and wanting undies?

DD is doing great with pee, she's consistently telling us she needs to go, and aside from nighttime is staying completely dry.  But...she won't poo on the potty, she won't even tell us when she needs to poo, she just goes and then we figure out that she's gone, even then we'll ask her if she's gone and she says no. 

And of course now she wants to wear the big girl undies - which we've been doing, but she's still pooping in them.  I don't want to go back to pull ups at this point, the big girl undies were a huge incentive for her to start peeing in the potty, but because of the poo issue I'm not sure what to do?

We've had issues with constipation in the past, but now she's on Miralax so we are keeping it soft for her and she hasn't had any constipation issues in a while. I'm just not sure what to do at this point?  Help!  Thanks.