Insufficient pre-stretch leaves too much material near the sheet plane and too little positioned for deep walls or multiple cavities. The final forming step then has to create the entire distribution at once, increasing the risk of webbing, bridging, and corner thinning.
Low bubble height is one cause, but shape matters as much as height. Air leaks, uneven heating, a tilted bubble, premature tool movement, and restricted air delivery can send material away from the areas that need it.
Correction
The bubble profile should be compared with the volume and location of the draw. Airflow and timing should be adjusted to create a stable shape before plug or tool engagement. Downholders may be needed to form separate material reservoirs for adjacent cavities.
Increasing pre-stretch blindly can trade webbing for apex thinning. The final thickness map and first-contact sequence should determine the setting. Repeatability should be checked after the tool and oven reach steady operating temperature.