Oh no! Usually the reason for uneven stitch lengths is to do with poor travel of the work through the machine. This can be for a variety of reasons:
The work is under tension and not lightly bunched so it will not feed evenly. Do not spread the work out but keep it a bit bunched with only the part you are sewing flat. Having the sewing machine set down into a table really helps here so if there is any way to do this I really recommend it.
The bigger the quilt the harder it is to move it through the machine to do straight line sewing. I prefer free-motion quilting for this reason as I am only moving a small area under the needle at any one time and not the length of the quilt from top to bottom.
The work may be sticking on the bulk of seams. Finer fabrics, good pressing techniques and also pressing seams open can diminish the bulk at seams and reduce or eliminate this problem.
Some sewing machine feet are very poorly designed and do not grip or feed the work well.
Some sewing machines are too light with too small a motor and simply cannot cope with the thicknesses and the weight of the work.
Altering the pressure on your presser foot can help. Too much and the work will not be allowed to feed very well.
The stitch length can be too short. Try around length 3 or even more to get things moving.
Heavy duty needles (and thread) make it difficult to penetrate the fabric and so reduce the ease of work through the machine. I work with 70 microtex needles for quilting (and often smaller 60 needles too - even on very large quilts).
Good luck with sorting this problem out everyone.