When I teach my students how to fix sewing machines I usually ask them what are considered to
be the most important components on a sewing machine.
As you can well imagine all sorts of answers fly around, but there is simply only two that I accept.
NEEDLES and THREAD and in both quality and condition count for a lot.
So the next time you have a stitching issue change your needle for a new one and check your thread is a good quality.