The glue holding the original paper strobe disk often dries out, resulting in a (partially) loose paper strobe disk. Hence, it sometimes is missing. Alternatively, Dual lovers of the 'polishing kind' sometimes ruin the paper by using excessive amounts of water while polishing.
Tags: Stroboskopscheibe, Papier Stroboskop, Strobe disk. 

Dual CS 701 Paper Strobe Disk on thick A3-paper