Just fix the band retainer. Sounds like it is either out of form or off location. You can bend it out for more tention, but I would recomend removeing it from the stock first by use of a small punch and drifting it out from the thur hole on the opisitside of the stock. Other wise you risk snapping off the pin that holds it in the stock, and then you will need a replacement. It may also just be the cutout is recessed too deep. The forward section of the retainer should fall just below the surface of the stock. If it looks like it is way back in there you can remove it and shim it out. But only shim near the pin, or the sping will not have any where to compress to. Or the lazy option, it you dont take it apart vary offten or dont shoot it much would be to push the band in place the lightly drift the retainer out enough it will catch the band. It will work for a while before it needs redone, but the shock from fireing and the tension will eventually work it back in the stock.