I am sorry to say that If the fortress is put together TOO TIGHT (Not enough of the white middle pieces put in) there is too much pressure on the comic and it can then split the cover and or pages . I have seen this MANY times even before CGC and it is why I stopped using them when I was a collector. Too many added and the book will slide anound as well.
Also, when we get a restored book sent to us in a fortress, we ALWAYS call the customer and explain that if we take it out of the fortress and there is arcyrlic color touch on the cover, it may stick to the fortress film touching the book and may come off once it is lifted off the comic . We always ask for an email to confirm that the submitter understands this and gives us a release to remove the comic from the fortress prior to grading.
These are 2 of the few reasons, when starting CGC, we stayed away from the fortress concept.
Oh, and BTW.......keeping a comic in a fortress long enough with enough pressure, presses the book, so please disclouse how long your comic has been in the fortress when selling it or you are not an upstanding collecter in the community