First, I added code tags for you ... notice that it looks much nicer now...
Anyway, Swing isn't thread safe. You aren't allowed to change values of Swing component outside of the event dispatching thread. This means that line 12 of your code, the clockLabel instance, assuming that it is an instance of a JLabel, may cause issues. This may also be the reason why you think that the threads are not running -- you may have frozen your GUI.
Henry