There is a possibility that the toner had a different make up. This may account for it but don't quote me =D . Using an iron I did on occasion have bad spreading of some tracks so there is a trade off on heat and pressure to resolve it I would think. It's all down to practice in the end to get that 'knack'.
I dread my laminator breaking........In fact I have a spare stashed away ready to modify and press into service :
I should say that from time to time there are minor imperfections that I have to use a sharpie to fix. But usually only a minutes work at worst......so far. But I do expect to have a total screw up some time that will require a full clean down and restart at some point.
regards
Fenrisulfr