Are these the automatic locking arms (rectangular black squares mounted up high on the outside, side walls of the slide)? If so... we had this trouble once, early on with our '05 Mountain Aire and your trouble may be the same as ours was. There's a magnetic switch that gets triggered by a small steel plate being pulled into place at the right point in the slide's travel. This triggers the circuit board so it "knows" that the slide is now in and starts the lock arm extension cycle. On our coach, the gap between the magnet and the plate was just the slightest bit too large, so the magnet switch didn't always get triggered. A couple of taps on one or the other side to move them closer together solved the problem... and it hasn't returned since. If I recall correctly (which could be a stretch, LOL!) if you look in the front-most compartment under the slide with the problem, you should see the arms and slide mechanism. Watch it as it retracts/extends and look for a spot where a plate passes something with wires (the magnet switch) and see if one or the other can be shifted closer to the other.
If that's not the fix, you may have a problem with your circuit board. You could try swapping the one with the problem slide with another one and see if the trouble goes away. On our coach, I think the slide controller circuits are mounted to the chassis rails, up high in the basement storage area. Try unplugging one and then try running the slide. If it runs... that wasn't the right controller... reattach the wires on that one, remove them from another, rinse and repeat. ;D
Hope that helps.
2005 Newmar MADP