You can not take a cast of the tire tracks by the gate, but can take the impression from the area near the post.
Realistically of course, CSIs would take multiple tire casts from all the areas possible at the crime scene to show that they are consistent- the game makers just decided that players just needed to get it from the post area and assume that the marks by the gate are the same
Also note: sometimes the pointer doesn't turn green, but you can click to get closer-it's an annoying glitch that happens sometimes, sometimes randomly to drive people crazy. Sorry can't do anything about that