Seal failure on flexible packaging

A seal fails when the jaws never made a bond, the sealant was contaminated, or the sealant layer was the wrong one for that temperature and dwell.

Checks

Look at the opened seal. A clean peel with no sealant transfer means it never bonded. Whitening or torn film means it bonded and then the film failed.
Check for powder, ink, or product in the seal area. Contamination is a channel you can see.
Confirm jaw temperature, pressure, and dwell on the real film, not on a lab strip of a different structure.
A laminate that is still giving off solvent will not seal cleanly. Smell and a retained-solvent check come before a hotter jaw.