Spotting can happen with many different methods. You do not lose that much blood with spotting, even though it might seem like it.
Still not working? You may have more luck on a pill with a slightly higher dose of estrogen, or one that provides estrogen during a different part of your cycle.
Try a different method: IUD; pill

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References:

1. FSRH Faculty of Sexual and Reproductive Healthcare. (Amended 2019). Progestogen-only Injectable Contraception. RCOG. Retrieved from https://www.fsrh.org/standards-and-guidance/documents/cec-ceu-guidance-injectables-dec-2014/