She could have been a very small hatchling. I had a hatchling last season who was not a twin but hatched out at around 35g. Of course, he didn't make it due to deformities but if she has been feeding on rat pinks or fuzzies then I could see her only being 50g if she hatched out small. You could try leaving her alone longer with the feeder. I have a bp that normally will not take f/t. Every time I feed f/t I have to simply put the feeder in the tub and I check the next day. It is gone by then. A fuzzy or hopper mouse will not harm your snake. You could try leaving it in for a few hours or even overnight.