If your working on a thick bit/seam/hump.. and the foot looks like its sitting on a slope,or foot jumps off the work as you try to sew it, you need to put a "shim" behind or by the side or your work to bring the foot level. You may have a few with the sewing machine, or you can use folded up cardboard to get the right thickness.. anything basically apart from metal! If some came with the machine, they just like small rectangular pieces of plastic in varying thickness's. It will help keep your line and your stitches evenly spaced as you have have set on the machine. You raise the presser foot, put your work in where you want to begin sewing, place the shim where you need it to keep the foot level with your work and lower the presser foot. You may not need it for the whole seam.
If your already on a seam and your coming up to another seam that is a lot thicker, you can also use a shim. As you approach a thick seam or other hump, when the needle is in the fabric, lift the presser foot and insert a shim under the presser foot, behind the needle to help level out the foot. I often use a sewing machine needle case as a shim when sewing over thick seams in denim. If you keep the foot level it allows the fabric to feed under the foot and over the foot dogs evenly and will prevent jam-ups .. when the machine decides it wont go over a hump because the presser foot is uneven and it doesnt press the fabric down onto the foot dogs.
Here's the foot on an angle, the work could slip out of the foot as there's not consistent pressure holding it down....
Here's a shim..
Here's the the presser foot with even pressure with a shim under the foot, behind the work...
Also if you dont adjust the tension for thick seams , even if its just for a few stitches, the top stitch will be too tight and it wont lock in the middle of the middle of your material.
A really easy way to test your tension for the fabric is to get two different colours of thread one for the top and bottom.. sew along the
bias of a scrap piece of the material your going to be using. Take the material in your hands about 3" apart and give the seam a good sharp and forcefull tug with a hand at each end of the seam.. like you mean to break it
If the top tension is too tight for the material it will have broken and left the bottom colour intact.
If the the bottom tension is too tight, the bottom colour will snap and the top colour will be intact.
If the tension is perfectly balanced, both will break!
A graphical explanation which has been my easiest to teach ppl about tension...is this