Is she refusing to latch onto the breast entirely or is she latching and then coming off and crying??
It might take a couple tries for her to get used to it but the sns is seriously a blessing. Well, out lactaid was
but they're the same thing basically, just lactaid uses plastic bags and sns uses a container.
Did you have someone show you how to use it, like a lactation consultant? If not, you should place the string so it lies flat against your nipple and stops just short of the tip of your nipple, definitely make sure it doesn't overhang because that will cause your LO to gag and have trouble getting milk out. Once you have it positioned right, place the tape just outside your areola. Also, make sure you try both breasts, she might just not want to latch onto one but be fine with the other (I speak from experience lol I now have one b cup and one c cup breast to prove it). Also, how high or low the sns is in relation to your nipple will change how easily the milk flows out, if I remember correctly.
I loved our lactaid. My LO did latch her entire first month (at 8 months we found out she had a severe tongue tie and a lip tie so we think that's why) and had to transition from bottles to nipple shields to get her to latch. Once she started latching without the shields, she started refusing her bottles, but I wasn't making nearly enough for her and her latch still wasn't great. We got a lactaid and the day before her first birthday was the last day we ever supplemented because using the lactaid in combination with herbal supplements helped me build up my supply so much
If she's not latching at ALL and you're really desperate like I was (and I know some people don't like this idea but it seriously worked for us) I would say try nipple shields. I don't know if it was just transitioning from one plastic nipple to another kind of plastic nipple is what she needed or what but we got those and she immediately latched for the first time ever. I cried I was so happy that day lol