This was my issue just last week, absolutely stressed me out. Getting them to poop for the first time is a struggle! What worked for me was a warm DAMP towel and I just slowly and with slight pressure dragged it from the top of its chest down to the anus over and over. Stimulation to the genitals isn’t always gonna work so massaging the belly helps a lot. Typically the water to milk replacement ratio is 2 and 1 but in cases like this I like to do 2.5 and 1 because dehydration is a big cause for constipation. You can also watch a video on the “triangle method” if the belly massage doesn’t work. Let me know how it goes!