Hoppers are WAY too small for your girl. That is likely your issue. Like you, I thought there was no way my snake could take larger prey, given the size of her neck. They will definitely surprise you.

I have trouble eyeballing the "widest part of the body rule", so I also check weight of the feeder. Rule of thumb is around 10-15% of body weight. So at 400g that would be 40-60g of food per feed. My normal girl Astrid is approaching a year old and is right at 400g at last weight. She gets small rats that are on the smaller side, around 60g in weight. I don't have access to weaned rats so I have the shop I buy from pull the smallest of their small rats for me, and I feed closer to every 10 days.