Anti-static film.

DTF transfer film carrying a surface treatment that lets static charge bleed away instead of building up, so sheets separate cleanly rather than gripping each other and the machine. PET is an insulator, and every contact-and-separation step in slitting, stacking and feeding lays charge on it that the film cannot shed; adjacent layers then pull toward each other until a stack of sheets behaves like one slab. It reads as a feeder fault, so the rollers and the pickup pressure get adjusted first, and the real tell is that it gets worse as the line runs faster rather than better. A double feed puts two sheets under the heads at once, and the wasted film costs almost nothing next to the print head it strikes, which is a machine down and a service call.