Step 1:
Make sure you unlocked the cable stopper. If the cable stopper is still active, cables should not be pulled out, nor can they be retracted properly.
Step 2:
The mechanism does not start automatically if you have unlocked the cable stopper. To activate it, pull the cable’s other (!) end until it reaches the same length (or as much cable length is left in the Cassette). When you release the cables, they will retract on their own. If there is still cable to be retracted, repeat step 2.