Wa'alaikumassalam sis! Congrats on baby sulung! Honestly, setiap baby lain-lain lah, some babies memang boleh accept certain formulas better than others, a few of my mum friends also had to try a few brands before jumpa yang baby suka, so don't worry you're doing something wrong, okay. Since baby is still very young at 15 days, I'd suggest checking with your pediatrician which formula suits baby best, sebab sometimes the tummy needs time to adjust jugak, or maybe the mix of BF + FM needs a bit of tweaking. Nan Ha and Nan Pro 1 are reputable brands, tapi kalo baby consistently reject Nan Pro, mungkin boleh discuss dengan doctor pasal switch formula or the feeding method. You're doing great, mama!