Vacuum lock occurs when a part seals against the tool and atmospheric pressure prevents separation. It is most common on broad smooth surfaces with few paths for air to enter during release.
The forming vacuum may already be vented at the manifold while a sealed pocket remains trapped beneath the part. Pulling harder can distort the part or lift a poorly secured mold without breaking that local seal.
Correction
A controlled air path must reach the locked region. Release-air ports, vents, shallow channels, or an appropriate tool texture can allow pressure to equalize. The sequence should open the air path before significant mechanical withdrawal begins.
Excessive release pressure can damage thin features or launch the part. The mold must be securely mounted, and airflow should be metered for the weakest geometry. A release agent has limited value when the dominant force is trapped pressure rather than surface friction.