The last st in a row is known for being really loose. Keep knitting a few more rows and it may improve. Also, if you use the backward loop cast on, the yarn moves along and is very loose at the end of the row. Try the knitted, cable or long tail cast ons instead and see if that helps. Or you could just CO one extra stitch and let that last on drop off instead of knitting it, then turn and knit the next row.
sue- knitting heretic