From the article:
And in the two decades since, the federal government says it has conducted more than hundreds of millions of background checks. With critical shortcomings in the system, though, mass shootings continue happening in the U.S.
“The weakness of the NICS system is talked about mostly in the wake of a tragic shooting, which happens more often than not,” says Stephen Morris, a former FBI assistant director for the Criminal Justice Information Services Division, which oversees NICS.
This seems about right. The system that does these checks has been denied funding, and research to be more effective.