Skipping stitches and knotting up underneath.

Hello,
Here are a few things to try.

* Raise your feed dogs if your machine has this feature.

* When you start to sew a seam, hold the upper and bobbin thread tails. Hold them back and out of the way as you sew your first couple of stitches. This will keep them from getting caught in your machine.



* Your fabric may require a different needle. Generally, heavier fabrics require larger needles and thinner fabrics, smaller needles. You may also need a larger needle if you’re sewing through many layers of fabric. And make sure to use a ballpoint needle for knit fabrics and a sharp needle for woven (or a universal needle for either). And make sure your needle is appropriate for the type of thread you’re using.


* Adjust the pressure of your pressure foot, it may be too light for your fabric.


* If your upper thread and bobbin threads are different types, try using the same thread for both. And use a good quality, brand name thread.



* Change your needle plate. Try switching to a needle plate with a smaller hole (a straight stitch needle plate).


Important Note: If you change your needle plate, check to make sure your needle aligns perfectly with this smaller hole before starting to sew. A misaligned needle could hit the plate and break, which could be dangerous. And make sure to change your needle plate back for zigzag and other wide stitches.

Hope this helps.