Present on the PTS and since the beginning, every cranes of the game seem to use the truck they are installed on, as a reference, to determine if a winch point can appear on any cargo we want to attach to.
So in many situations, the boom of the crane and the green limit circle are more than close enough of a cargo but no winch point appear on the cargo.
To make it appear you have to get the truck closer to the cargo.
Every crane should use the tip of its boom as the reference to determine if if a winch point can appear on a cargo.
On the pictures below you can see that the container is well inside the green limit crane circle, yet no attach point appear on the container.