i would suggest posting your actual code.
also, read this:
http://www.php.net/manual/en/function.move-uploaded-file.php
it tells you the conditions underwhich it fails. perhaps you can check for those conditions first to perhaps intercept them and/or figure out what the problem is.
as for errors, do you have error reporting turned down or turned off? you could try temporarily turning it all the way up while you test this so that perhaps you can get more information.
another possiblity is that you could write some pretty involved code using lower level functions (e.g., file_exists(), is_readable(), is_writable(), is_file(), copy(), rename(), etc.) and check for the various conditions you need to be true before trying to move it. this is what i do.
