Jefferson Nickels are known for being struck weakly on the steps, the alloy of the planchet is hard on dies. Plus the design caused some issues with the die adjustment in order for both sides to be fully struck. Often the die wasn’t adjusted exactly to accomplish the task. So it is common to see weak steps on the Monticello Building and die fatigue and wear from use.
There’s two designations for full step or FS nickels depending on the grading service; NGC uses 5 FS and 6 FS but PCGS on uses FS. PCGS would label it like this MS65FS and NGC MS65 5FS or 6FS.
You also have 1938 Wavy Steps and the Steps of 1940 that are straight and more parallel. See images below.
As you can see from the images above a 5FS can have a weak and partial lower step but a 6FS must have all steps fully struck. Contact marks can impede but it is the complete strike that is most important.
Proof Jefferson Nickels are struck more than once and will always have full steps so they’re not labeled as such. Only business strike Jefferson Nickels will receive the FS designation.