Did you remove the antenna and disassemble it at the bench? I got a replacement mast off EBay. All you need to do is remove everything except the motor itself, clean each part with degreaser or whatever, lube with suitable grease (white lithium works well), and put it back together. As you take off each part, place everything on the bench in a line in the order of removal. That way, correct reassembly is much easier, or take a few pictures as you go. Pay attention to the washers for the main reel case on the post, and don't loose the tiny C-clip.
When it comes time to put the new mast in, feed the cord down the tube and into the toothed cog that pulls it up and down. Use a 12V source to let the motor pull it the rest of the way in after everything has been put back together.
IIRC, the wire colors on the antenna plug (not the car harness plug) for bench testing are as follows:
blue/yellow stripe, yellow, red/green stripe all connect to 12V+
white/black stripe to 12V-
Touching either black or red/white stripe to 12V+ will extend the mast, removing 12V+ will retract it.
If you do this at the bench, you'll be able to see why it didn't go up all the way if the problem is inside the antenna. If it works flawlessly off the car, but not after installation, then I'd say you have a wiring problem, either in the harness or at the radio trigger wire...the one that connects to the black or red/white stripe pin.
BTW, if the mast cord is in correctly, and there are no missing teeth, or broken parts, it can't 'slip' since the black idler will keep the cord teeth meshed tightly against the big toothed 'gear'.