Anyone who has ever made a bag has probably encountered the same problem: eventually, usually on the last seam, you have so many layers of fabric coming together at the seam line that your sewing machine throws a total fit and the final seam - usually a very visible one! - gets all wonky as you try and force the machine up and over the layers.
Fortunately, there is a VERY easy solution to this problem, and it will cost you nothing. Sure, you can buy little plastic doodads (one is amusingly name the 'Hump Jumper'), but why bother, when a simple piece of white paper will do the job.
So here's the problem. I'm sewing along, and I arrive at the point on the bag where the strap attaches. All of a sudden we're going from four layers of fabric up to about 12, 14 layers, including two layers of fleece! The difference in thickness is large, and most sewing machines do not cope well with sudden height changes like that. They do a lot better if the presser foot can somehow remain level.
So here's my easy and free solution. Fold up a piece of paper to whatever thickness you need to raise the presser foot to the same height as the seam. With the needle down in the fabric, raise the presser foot and tuck the paper underneath.
Then, lower the presser foot and start sewing.The paper will keep the presser foot level as it transfers onto the seam, and your stitches should stay nice and even and your machine won't struggle! It's as easy as that!
My machine doesn't cope well with sudden height changes coming off of a seam, either, so I do the same thing on the other side. Stop with the needle down, tuck a folded up piece of paper underneath the presser foot, sew off the seam, and then you can remove the paper. If the seam is narrow, you might need a piece of paper on either side - whatever you need to do to keep that presser foot level! Just remember whenever you need to add or remove the paper, stop with the needle down, raise the presser foot, and make your adjustments. And don't forget to lower the presser foot again when you are ready to start sewing!
Here's the bag I was working on when I decided to photograph those steps for this tutorial. Just needs a strap and it's done.