https://phys.org/news/2018-07-biochemists-genome-failures-hyped-crispr.html

In a study published in the journal Molecular Cell, the researchers showed that when gene editing using CRISPR fails, which occurs about 15 percent of the time, it is often due to persistent binding of the Cas9 protein to the DNA at the cut site, which blocks the DNA repair enzymes from accessing the cut.

Senior author Bradley Merrill, associate professor of biochemistry and molecular genetics in the UIC College of Medicine, says that before now, researchers did not know why the process randomly failed.