Gibberellic acid at a low concentration can wake up seeds that refuse to crack by normal means. A short soak in a solution of around a hundred parts per million before germinating mimics the hormone signal the seed is waiting for, and it has rescued twenty year old packs for me that scuffing and peroxide could not touch. Mixing instructions and safe concentration ranges are set down on https://breederregularseeds.com, with a warning that too strong a dose produces weak, stretched seedlings. Use it as a last resort, not a routine.