Does the tether come out a few feet then stop?
If the tether is not extending the full 12 feet then it could be that the tether has come off the internal spool and is caught in the mechanism.
We suggest you remove the 4 screws that fix the end cover in place. This will expose the internal spooling mechanism.
You may be able to see that the tether has come free of the spool.We suggest that whilst you have the end cover removed. Try and pull (slowly and gently) it out all the way ... Once it stops you should be able to see where it is getting caught.
You then may need to use you fingers to help guid it back onto the spool.
Utilise the tether help information ... and this should help get you sorted.