It will not release
Updated
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Straight walls with a deep fill
The worst combination, and also the most attractive one in a photograph. The wax grips along the full height and there is no taper to help it out, so the customer breaks it up in place.
A mouth narrower than the body
Any inward curve at the shoulder turns extraction into leverage. The wide 89-400 and 70-400 necks on this shelf exist precisely to avoid this, and they are 112 of the 224 containers.
Cooled too fast
Chilling can set the surface before the body has contracted, so the wax ends up stuck and cracked rather than pulled away cleanly. Slow, even cooling at room temperature usually releases better.
Too soft to hold together
A heavily loaded or soft wax breaks rather than lifting, which looks like a release problem and is actually a formula one. Firmer wax in a tapered tub is the combination that behaves.
Ask what the container is for
Several things on this shelf are candle jars, and a candle jar never has to give its contents back. Reading a jar as a melt cup because it is the right size is how this starts.
Empty one by hand before you commit
Set overnight, capped, then opened and emptied the way a customer will. If it needs a tool, pick a different container, because no instruction on the label fixes it.
Taper beats everything else here
If one property decides this, it is whether the walls open outward. A tapered PP tub releases a set melt with a push; a straight-sided jar of the same capacity and price does not, and the two listing photographs look equally good to anyone who has not poured either.