Actually true wheels can still have flat spots too.

Ever seen what happens when radio static cuts in at the wrong moment? My buddy worked a job where they only used radios, just like your Phoenix site. One afternoon, the operator's headset filled with sudden noise and he missed a stop command. The load swung way too close to a guy on the ground, everyone froze. They got lucky that time, but now his crew always uses hand signals as a backup. Relying only on gear seems fine until it isn't, and that split-second mix-up is all it takes.

Adjust it while riding up a mild incline.