Each arm has a prox. When an arm is in the full "out" position, the proxes should be off. As the arm sweeps inward, the prox should turn "ON", and as they close to center, the prox should again go "OFF". Code 17 indicates that an arm prox isn't turning on when expected. best way to determine which one would be to look at the 4 digit diagnostics if you have them available. 8813 is Rt Front, 8814 is right rear, 8815 is left front and 8816 is left rear. If the arms can't move in, that would cause this code.
For the arms not coming in, trailing air refers to the direction of travel when those arms are not leading. In other words, when machine is travelling rear to front, rear trailing air is what controls them. When the machine is travelling front to rear, it is "rear air" that controls them. Depending on how the machine is setup, high rear air can also come into play, but usually "rear air" should be enough to bring them in. If you believe that valve is functioning properly, try dropping the air off the machine, and move the arms in/out. Is there any binding? Are the arms level? Are the pivot pins in the air cylinder loose? Are any of the air lines going to the rear arms pinched? Anything that could mechanically restrict the movement can be contributing.