Is it possible that your last bound off stitch really is bound off, but looks bigger than the others and kind of sloppy?
If so, here's how to neaten it up:
When you have one last stitch is on the left needle (with one on the right needle), slip the one on the left needle back to the right needle.
Use your left needle to pick up the stitch below the last stitch on the row below. Bring it up onto your left needle. Then slip the stitch that you put on the right needle back to the left needle. Knit it together with the stitch you picked up from the row below when you bind off the last stitch.
Hope that helps!