Interesting wall wart issue

[GibsonGM]

Hi,

Here's a question looking for an answer.  I have an easy vibe, of course it likes to go thru batteries.  I tried a little experiment with a wall wart, just hooking it up to the battery clip...PS is 9V, 500 mA, seems like enough ampacity for this one!  It worked fine with the short leads I have on the wall wart. 

I extended those leads to like 8 feet, and tried it out again - and it doesn't work!  The power LED's go on, and with some fiddling I can get the opto LED's on steadily, but no oscillation.  The no-load voltage is 14. something, as expected, and yes the polarity is correct.  These symptoms seem like the effect is starved for current...but it works with really short leads, kind of weird.  Anyone know why that might happen?
Just wondering.