Hmm... I think those should work, but I haven't used them myself. When I need to display debug info, I usually do this:
<?php
drupal_set_message('<pre>'. print_r($variable, TRUE) .'</pre>');
?>You can also put debug benchmarks in the code, like dsm(1);, dsm(2);, etc. at various points in the module to see what parts get executed and where it goes awry.