1) You can, and maybe even should, pry the back off a pot, by GENTLY lifting the tabs, and see if there is anything obstructing smooth rotation of the pot. Understand that thereis a "stop" built into all pots to keep them from rotating past the 7:00 and 5:00 positions. It is physically possible to re-install the shaft/wiper assembly of larger pots like the one Dave/Greco showed "incorrectly" such that the shaft/wiper bangs up against the stop early. So check that out and make sure that's not the issue.
2) Distinguish stiffness resulting from a shaft that does not want to turn, from stiffness resulting from a control knob pressed down against a hex nut so that the shaft is prevented from turning. Does the pot rotate smoothly when there is no knob on it?
2) Distinguish stiffness resulting from a shaft that does not want to turn, from stiffness resulting from a control knob pressed down against a hex nut so that the shaft is prevented from turning. Does the pot rotate smoothly when there is no knob on it?