There is a difference between tightness and tension in a muscle. Tightness you can stretch. Tension can be thought of as big knots in the muscle. No matter how much you try to stretch a tense muscle, there is limited chance of the tension dissipating.
Think of an elastic band that has a knot in it. To get rid of the knot, it either needs to be cut, unravelled or given a high amount of repeated pressure until the knot breaks. Try stretching the elastic band as far as possible and the elastic will probably snap, but the knot will still be there.
The same applies to muscles in our human body. Over-stretch a tense muscle and it will likely tear, but the tension will still be there. Alternatively, apply massage or dry needling to release the knots and tension areas so that the muscle CAN relax and then you CAN stretch it.