Apple MagSafe Battery Pack Not Charging

I have a Apple MagSafe Battery Pack that I purchased for $100 and it’s simply not charging. It shows that it’s charging on my iPhone 13 Pro, but the battery percentage hasn’t moved up. In fact, it’s only been going down when I charge it. The fact that I payed double for this charger that I could’ve gotten elsewhere for $50 is upsetting, considering the fact that it’s doing absolutely nothing for me. Is this a defective battery pack or is there something that I can do to fix this issue?