The bobbin case and upper tensions are adjustable for a reason. That is so you can take control of your tension when sewing on many different fabrics and applications. The bobbin tension will naturally loosen with the vibration of the machine. It's up to you to tighten it yourself or pay a tech mega bucks to do it for you. If it's nesting on the bottom, it's usually the top tension that has an issue. Unless.... there's a burr on your hook, or your bobbin is bent or damaged, or the timing is off. If the upper tension isn't tightening, there could be lint in between the disks. Good Luck! I hope you can get your machine sewing.