2-3 week old kitten won’t poop

Hello everyone! I just joined this forum looking for some help or advice. Today, my partner and I adopted a kitten that seems to be 2-3 weeks of age after someone brought it to where we do some volunteer work. We’ve been feeding the kitten every 4 hours, burping it after each feeding, and afterwards stimulating the tummy/genital area for potty time. She’s managed to pee but not poop. We’ve tried massaging for more than a minute but to no avail. My partner is going to call our vet on Monday to see if we can schedule an asap evaluation, but I at least want to make her poop before we take her to the vet.

Does anyone have any advice on how to make her poop? I am truly worried something wrong could go wrong and somehow negatively impact our kitten.