Are lots of people skipping their second COVID vaccine dose?

On May 28, 2021, Pennsylvania go to 70% of the adult population partially vaccinated. The fully vaccinated number was only 52.7% at that time. I figured that meant in 3-4 weeks the fully vaccinated number would reach 70% because the delay between the first and second Pfizer shot is 3 weeks, and the delay between the first and second Moderna shot is 4 weeks (J&J gets you fully vaccinated after one shot--and those three, Pfizer, Moderna, and J&J are the only approved vaccines in the U.S.) Which would mean our state could be approaching that magical (perhaps mythical) herd immunity number by early July.

It's not 3.5 weeks later and the percentage of fully vaccinated adults has barely moved. Today the PA Department of Health web site says 54.2% of Pennsylvanians age 18 or older are fully vaccinated. Are that many people not showing up for their second Pfizer or Moderna shot? If that's the case, why isn't is a huge issue?