The tool that you have appears to be what we call a jointer or joiner. It is intended to get one side or edge perfectly flat and square to it's adjacent side. It is NOT a planer/thicknesser.
Was the surface that you ran through your jointer straight and relatively flat before you cut it? If it wasn't and was cupped I would expect it to only get cut at the ends. The jointer removes the parts of the board surface that touch the table to make that surface of the board straighter. It may take several passes before the jointer cuts an even amount of wood off of the full surface, but once it has that surface will be perfectly straight and smooth. Now rotate the stock 90 degrees and place this freshly cut edge against the fence. Now, with each pass you will be straightening and flattening the second side of your stock, but also making the first and second sides perfectly square with each other (if the fence is set correctly).
A second tool, called a thickness planer, is then used to make the two remaining sides of your stock parallel with these first two sides and it can also be used to reduce the thickness of the material to whatever is required by making many passes with reduced cutter settings each time. Although it is possible to straighten and smooth all four sides of your stock with your jointer, it isn't possible to get the opposite sides perfectly parallel with each other using only a jointer, unless the jointer is one of the few jointer/planer combination tools that are on the market. I can't tell for certainty what it is from the photo as I have not seen one of this model before.
I hope my explanation has helped.
Last edited by CharleyL; 02-14-2012 at 08:46 AM.