As much as I detest the word ‘root cause’ and as much as it is almost never a single cause but rather a set of mechanics, there are a few situations where the singular word ‘cause’ applies well enough. One example is bagginess. The great great majority of bagginess is simply variations of basiswgt/caliper/thickness. The mechanics are that the web stretches more (and thus yielding more) on the high gage areas. It does not matter that this is the best you (or your supplier) can do. The winder is the ultimate judge here. It does not matter (much) that the winder makes the defect happen because it can not unmake it by playing games with TNT's or taper. It does not matter that your scanner or test lab can’t always see the variations. No surprise as the threshold of pain can be on the order of 1-10% thickness variation and variations that small can sometimes be difficult to measure. At 1% you have few complaints (except foil where that is not good enough) and at 10% you have few customers (except for garbage (bag) material that is oscillated 360 degrees).
On thick materials we often can measure these thickness profile variations well enough and note the corresponding baggy lanes. On certain materials, mainly paper and some harder grades of film, a roll hardness profile will ‘see’ the variation far better than your expensive scanner because it effectively measures (samples, averages) many hundreds of layers with every hit. Here again the connections between raw material thickness profile variation and wound roll profile and bagginess profile is profound and clear.