If you're trying to do a multi-table select by using the simple syntax (WHERE a.col1 = b.col1 etc.) and it's not working, check to see if you have any null values in any of the columns you're trying to select. That can sometimes cause that sort of query to fail.

If that is the case, you need to use a Join instead.

This has tripped me up on more than one occasion, but Join (usually a Left Join, with the most important table listed to the left, i.e. first) generally sorts it out.