Joule thieves?

I am trying to make a joule thief according to many websites and such, but it never seems to work, even if I follow the instructions. The joule thief I'm trying to make is one of the simplest, with a 1K resistor, transistor, tapped coil, etc. I think it may have something to do with the inductor, even though I've tried a whole different types. I got it to work once, though, when the 1K resistor was replace with a 10K resistor accidentally. I heard somewhere that the resistor should be around 1K up to 3K. Any higher and efficiency goes down. Is that true? And why doesn't the joule thief work when I use a 1K resistor instead of a 10K resistor?

P.S: I have made many other joule thieves, my first one being from http://joulethief.com/kit.php, and my favourite, the supercharged joulethief from http://quantsuff.com/LED2.htm.