I got a strange distribution of insert size as analyzed by Qualimap on the BAM file. Not totally aware of the library preparation procedure yet, but this is a PE-sequencing on NextSeq500, libraries were made with NEBNext Ultra II kit and size selection beads. The median insert size was expected to be around 300bp. The problem is that we got not only a much smaller median insert size (190bp), but also a non-symmetrical distribution. What could be the reason? Is this a concern? Thanks for any help.