Uneven stitch and feed make the form of a working product distort and fabrics not sewn firmly. If you’ve spotted the problem when sewing, you may check the machine if one of the following happens, which are the reasons behind the issue:
The quality of the thread is poor
Threads of low quality are easy to form lint and not able to sew fabrics in a uniform manner. Choose a durable thread type with low shrinkage that forms little lint.
Quality threads do not form lint easily, are durable and of low shrinkage.
The bobbin case is threaded wrong
When the bobbin case is threaded wrong, the upper thread and the lower one do not work in sync, forming uneven stitch or uneven feed among layers of fabric. Check the bobbin case and re-thread if needed. The thread should pass through the thread side slot in the case before being brought up to the needle. Check out the details of inserting the bobbin here.
Insert the bobbin in the case by passing the thread side slots.
The fabric has been pulled
It is possible that when sewing, the fabric is stretched when you try to make it pass through the machine. Do not pull on the fabric while sewing. Let the feed dogs pull the fabric under the foot instead.
When sewing, let the feed dog pull the fabric instead of pulling it yourself.