Make sure the cable is free to pull the mechanism, especially when you say the spring in the derailleur won't pull the shift lever back when coming back from the outermost position where the tension is the greatest.

I just had that problem last weekend, and when I looked, my cable goes down the bottom of the frame and comes up around the bottom bracket. I had let the kickstand shift and it was jamming the cable.

Also, I found it hard to push my derailleur out to the farthest position with fingers. I used a screw driver as a lever to move it out, and then I held that position with some scrap wood around the lever.