what am I doing wrong?!

I tried to culture the yeast from a few different bottled conditioned commerical beers, and each time it always gets infected or something and I just dont get it!

What Im basically doing is making a really really really tiny beer (volume wise), pitching in the lees in the bottom of the bottle and then sticking my airlock on top. Seems what i do when i actuslly make a batch of beer.but then each time there develops some gross white film on the top and smells sour so its obviously infected. does anybody have any idea why this isnt working?

Are you actually fermenting in the bottle, or pouring the dregs into a sanitized container? Even if you're pouring it out into a new vessel, make sure you flame the mouth of the bottle, to keep it sanitized.

i clean and sanitize a 1 gallon demijohn, and then add the small amount of wort and the yeast from the bottle into the demijohn and fit it with an airlock. i clean the demijohn in the same fashion as i would my carboys and fermenters

That's the same set up that I've used for growing up either from commercial or my own bottles, and it worked ok.

Assuming there's not something clinging on the equipment and spoiling so needing a mighty soak to kill off, have you tried growing from a young-ish homebrew to test the process? I figure there'll be more yeast and of a known age and provenance in an average homebrew bottle.

Is your water boiled and cooled? Is it covered while it's cooled? How long between finishing the bottle and using the dregs? Does this white film appear only on you bottle culturing attempts? Do you drink from the bottle or pour into a glass?

the water is boiled, as soon as its ready i open the bottle and dump the dregs in, and i dont drink from the bottle i pour it into a glass. the ponly thing is no i didnt cover it while it cooled. maybe thats whats getting me. and yea its been commerical bottles not homebrewed