The slide control is two positions on the same geared slide and the first position is just 4WD so in order for the diff-loc to work it has to slide into 4wd first. That being said the "step-one" position for the actuator isn't working. That's most often from a bad switch but also a bad connection. It can also be a problem with that actuator internal electronics...but check the switch first. Work the button a few times...it may have got water in it.