Additionally, many doctors have said that 2 weeks between shots was too quick and would not be enough time for the first shot to "primer". 2 weeks acted more like a single larger dose than 2 spread out doses.

Edit: My memory failed me, the spacing was 3 weeks and 4 weeks (Pfizer and Moderna respectively). Point still stands though.

Ref: https://www.medpagetoday.com/opinion/marty-makary/94315