A pre-stretch bubble should align with the intended material demand. When the apex leans or shifts, one side receives more material while the opposite side enters the draw already stretched.
Uneven sheet temperature is the most common process cause. Drafts, off-center loading, unequal clamp pressure, residual extrusion orientation, and asymmetric air entry can create the same shape. An asymmetric part may require a deliberately offset bubble, so geometric intent must be distinguished from uncontrolled drift.
Correction
Bubble shape should be photographed or measured from repeatable reference points. The sheet can be rotated to test orientation effects. Heater balance, air inlet position, clamp parallelism, and sheet registration should then be checked.
The process should not be centered by changing plug position until the bubble cause is known. Otherwise one error may be used to compensate for another, leaving the thickness profile sensitive to material or temperature variation.