Hi Patrick,
If they are drawn from a hollow gather, the tube was cut or chopped into bead lengths (Karlis Karkliins, "Beads" 1993 Vol.5) and their forms altered by heat rounding. This method of heat rounding is called "a speo". It could be that some of these overheated or were not allowed to cool sufficiently and in turn collapsed or sagged. My shot in the dark!
Thomas, always learning!