I've gone through this issue at length with both my supplier and 3M when this had me reprinting a reprint of 5m x 1.2m.
Basically, it's pressure marks. Here's the conclusion. Basically, if it's at the end of the roll, it's due to the roll expanding and shrinking during shipping. This makes the roll 'tighter' in the center if that makes sense. The pressure is what creates the mottling.
On the other hand, if you see pressure marks which resemble stripes, going in a perpendicular direction to the width of the roll, from the outside of the roll - this is the impression left on the roll from the corrugations in the shipping carton. This happens when one of the end plugs has been crushed in transit, resulting in the roll to rest on its side heavily on the corrugated packaging material. You will notice as you unroll your film, that there will be a striped mottle every revolution of the roll in the exact same spot.
The latter falls on the shoulders of your supplier. Look out for broken/cracked plugs and the such, and do not accept delivery and send it back. More than likely it has the striped type of pressure marks.
If it's the type that is towards the centre of the roll, there's not much that can be done about this unfortunately (words of 3M) - it's just one of the things that can happen in transit if it goes from a hot container to a cold area / vice versa etc.
Like Colorado said, post heat gets it out, it's just a big PIA. When it's solid dark it's even worse because any inconsistency in your post heating will also show up as a varying level of gloss
Good luck getting anyone to pay for your reprinting... but do try to get a free roll or a heavy discount on your next roll.