Would it not retract at all, or just from the ignition being turned on?
Mine stopped working from the ignition, but still worked fine from the door. I found a grounded diode/resister inline with the ignition's wire that had broken. I bought a replacement for it and installed it, and that fixed the problem. I think it is a voltage limiter to keep the 14+ volts from the alternator spiking the step's electronics.
Here is a link to the thread where I was trying to figure it out, and  finally got it repaired.
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/28038583.cfm