Sometimes the pattern is not printed on the grain. You are probably on the grain. Sometimes my fabric is 1-2" off when I line up the selvages, usually there is a longer and shorter side, [on the selvage] so hold up your fabric, align your selvages. That should be pretty straight. Trim and then cut a strip, lay it out and see if it has a whacky curve on the fold. If it does, adjust again. There are several good books on this problem.