When I get a call about a bubble that won't hold steady, the operator on the phone usually wants to talk about the resin first. I want to talk about the air ring first, because that is where most instability actually starts.
Uneven air ring cooling
The air ring's job is to cool the bubble evenly all the way around. If the ring is out of balance, more flow on one side than the other, dirty or partially blocked outlets, or just misaligned after a maintenance job, the bubble cools faster on one side than the other. That uneven cooling makes the bubble wander toward the side that is cooling slower, and once it starts wandering, it gets harder to control with every pass. Walk around the bubble and look at it from multiple angles. If it is leaning or breathing unevenly, check the ring before you touch anything else.
Frost line height problems
The frost line is where the bubble transitions from molten to solid, and its height tells you a lot about whether your cooling and output rate are actually matched. A frost line that is too low usually means you are cooling too aggressively or running too slow for the setup, and the bubble can get stiff and unstable. A frost line that is too high often means the bubble is staying molten too long, which leaves it soft and prone to swaying or necking down unevenly. Frost line height should be consistent and repeatable, not something that drifts run to run.
Draw ratio pushed too far
Every line and resin combination has a draw ratio it can handle reliably. Push blow-up ratio or draw down too aggressively chasing a thinner gauge or a wider layflat, and you are asking the bubble to stretch further than the resin and the cooling can support at that output rate. The bubble gets thin-walled and unstable before it ever fully sets. If instability showed up right after a gauge or width change, that is the first place I look.
Drafts in the plant
This one gets overlooked because it has nothing to do with the line itself. A door that gets propped open, a fan pointed the wrong way, or a seasonal change in plant airflow can introduce a draft that hits the bubble unevenly, especially on tall towers. If your instability is intermittent and does not track with any process change, check what else changed in the building around the same time, not just on the extruder.
The short version
Start with the air ring, because uneven cooling is the most common root cause. Then check frost line height and draw ratio against your normal baseline. Last, rule out drafts in the plant if the problem is intermittent rather than constant. Most bubble instability problems get solved by working through those four in order, not by guessing at a resin change first.