Why is top seam straight but bottom isn't?

I've noticed lately that when I sew, the top seam is nice and straight, but when I lift up my fabric, the bottom of the seam is sometimes crooked or parts of the seam are crooked. Does anyone know why this is? I would think that the bottom should match the top. This normally wouldn't bother me because I'm usually stitching from the top, the side that is going to show. But sometimes, I have to sew from the bottom side, like when I'm sewing curtains and I'm hemming them and I want to topstitch along the edge of the side I'm hemming. So then I want the stitching on both sides to look nice and straight. Does anyone have any suggestions on how to fix the crooked stitches?

I would play a little with your tension and make sure it is just right.  Also, try re-doing your bobbin if it isn't a drop in.  And as always, try changing your needle.  It is AMAZING what changing your needle can do and what can happen with a dull needle.  Crazy really.

I agree with changing the needle & trying a new bobbin.Is it all the time or just sometimes that this happens?

I did try changing the needle and rethreading the bobbin and needle thread. That's what I always do automatically if stitches aren't looking right. I'm wondering if it's the thread. I notice that it seems like this happens more when I use polyester thread and not cotton thread. I use mostly Guterman thread. I sometimes use Mettler thread if I buy it when it's on sale. It's a lot nicer, but kind of pricey, so I always buy Guterman (love those 40% off coupons at JoAnn's!).

I could try to adjust the tension, but I hate to mess with that unless I absolutely have to (especially when the stitching on the top looks fine).

Some machines just have problems with ceratin types of thread. It does sound like a bobbin tension adjustment but if the other thread works than I would stick with that type.

It could also be the fabric you're using.  I have noticed with some coarser cottons, not necessarily cheaper cottons, there are often crooked bottom stitches.  Try another type or quality of fabric and see if that makes a difference.  You might have to adjust your tension for those fabrics in order for you to attain neat stitches on both sides.