As you said it start to happen recently, I would check nozzles for those cavities. Check if they are not damage or plugged by something (metal chip...)
If those cavities are filled last, the preform have not enough time to be conditionned to the mold temp, so the bottom is too hot compare to the rest so it will lift easier, which will stretch more the melt. But as it happen on one station only... it's a bit strange.
First, try switching station for these core rods for these cavities and see if the problem follow. If so, your core rods have something. If core rods have a seperate neck ring, check if they are in position. If they've slipped, the bottom of the preform will be thinner.
One question, does your core rods are chromed?
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