Vacuum bags have a reputation for being finicky, but most of the frustration comes down to two fixable habits: a rushed seal and the wrong rolling direction.
Start every bag with the valve fully open and the seal pressed shut along its entire length before you begin rolling - a single unsealed inch will let air back in over the course of a flight. Roll from the closed end toward the valve in one continuous motion rather than several short pushes, which lets trapped air escape evenly instead of pooling in pockets.
For anything with structure - a stiff jacket collar or a shoe - fold it flat before it goes in the bag. Vacuum compression flattens soft fabric well but can't do much for shapes that don't want to lie down.
