Congrats on finishing your machine and making some cuts. I have to agree with MCYR, you need to do some squaring on your gantry. I clamped a scrap piece of plywood and then made a couple of short cuts on the x axis and then a couple of cuts on the y axis. I did this in a pattern and distance that would match up with my L square. I put the square on the plywood to see if the X and Y axis slots were squared up with each other. If not, I'd adjust one of the screws and then cut again until it was square. Mine is within 1/32 inch in 2 ft.
My Solsylva machine (my first cnc)
http://www.cnczone.com/forums/showthread.php?t=44336