Do you live at a desk? Work a physically demanding job? Carry tension all the time?
Anytime a body part is overworked and over-tight it's more likely to become stuck. The body will detect chronically lower oxygen in the tissue, assume you need a scar, and deploy baby collagen cells to the area for structural support. It's like glue gets poured down into the muscle, but when this happens you will feel weaker and less flexible in that area. I find that instead of a singular "knot" floating in tissue, what is more likely is that the knot is sticking a muscle to another muscle, or fascia, or to a nerve.