When you advance the film, it should STOP (lock the advance) with a number in the little window at the back. Then you press the black button on the back of the camera, opposite end from the viewfinder, to release the shutter. It makes a slight "click".
If you don't have the cassette fully seated on the camera, the hook may not engage the framing hole in the film, meaning a) the film won't stop at the correct location, and b) the shutter won't be cocked even if you manually stop winding at the right place. If the hinged back cover didn't snap into place, the film isn't seated, and if it pops off while you're advancing, the film will unseat (tolerances -- these cameras only sort of fit the film, it appears).
What I've taken to doing is holding the film in place when I advance, by squeezing the back (as if compression the camera back to front), then continue holding in place while I compose and release. Yeah, it's awkward, but less annoying than wasting half you film and getting that stupid little slip of paper in the envelope with your prints telling you how to avoid problems with your next roll...
