The possible causes are:
(1) 30% ethanol instead of 70% ethanol is used in DNA purification with magnetic beads; or
(2) forgetting to add magnetic beads after digestion. The workflow does not require users to change tubes, nor to remove magnetic beads from previous steps. One can easily tell if magnetic beads in each purification steps are added or not by checking the volume; or
(3) forgetting to add Stop Buffer after digestion, especially when handling a large number of samples.