is there a trick to installing "lock and fold" flooring?
I can think of a few possible problems:


The floor wasn't flat to start with.  You typically put down an underlayment of some sort, which will deal with some floor imperfections, but it's not 100% perfect.
Something managed to get under the flooring when you weren't looking.  (those little plastic things yout put against the wall being a prime culprit, but those are pretty obvious when it's happened).
There's something in the grove of flooring.  As part of the manufacturing, a little wood chip might get in the track, which prevents it from seating correctly and locking down.


Odds are, it's #3 -- it doesn't take much.  Inspect the board that you're trying to place, to see if there's any imperfections in the board along the grove, then take a look at the place you're trying to lay it into.  One the last floor I helped installed, we got in the habit of looking at all edges of the board before laying them, and pulling off any bits that were flaking that might cause problems later.  We also found that putting the board in the groove, then running it back and forth a few times would make them more likely to sit down on the first try (I'm not sure if it knocked out debris, or ground it down so it was small enough to not be a problem).