What is the distance I can put between a 20watt solar panel and a controller and still have enough current to make it work
Congratulations on doing it properly as you can in your situation, on adding an extension in a competent manner and in thinking about other ways to achieve your requirement.

Fortunately, it is likely that you have done something wrong with your extension cable. I say "fortunately" because it shoul be possible to make what you have done work. 

Perhaps the polarity from panel to controller has been reversed when you added the extension. Check the polarity with a voltmeter, with the cable connected to the panel and disconnected at the controller.  

12 gauge wire from panel to controller should be VERY adequate and cause no problems at all.  Resistance is under 0.1 Ohms and voltage drop at 20 Watts is probably around 0.1 - 0.2 V.