If it doesn’t crack, and you re-apply a new scratch in the same area, the strain left there from the first attempt will cause any subsequent crack to drift leaving a useless crack.

There are three options:

1. Create a new scratch a reasonable distance (1–2cm) away from any latent strain and try again.
2. If it’s small tubing, flame anneal the entire area of the scratch, let cool, re-scratch, and re-attempt the crack.
3. If it’s medium or large size tubing, (over 15mm), oven anneal the tube before re-scratching, or the resultant crack will tend to drift erratically.