Is one of your hamstrings longer than the other? And I mean left leg to right comparison, and not biceps femoris compared to the semis. How would you know? One way is to film yourself from the side, or watching in a mirror, while you’re doing a single leg good morning. If you can bend over less on one side then the other, there’s your answer. Also, pay attention to the feel of the test. If you can go as low on both sides, and one of the legs in the hamstring area feels tighter, then that also means it’s probably shorter. How to fix it? Either use that same single leg good morning, and try and reach deeper, very gently, as explained in this video, so let thy test be thy exercise and thy exercise thy test, or try doing two leg good morning, and only focus on the tighter leg, and adjust the deepness to that side. Eventually you’ll lengthen that shorter hammie and then you’ll go deeper and start feeling both stretching at the same time, then you’ve got it.
Why do we get that imbalance? Probable due to previous injury, like kicking something and over swinging and over stretching the hamstring, causing some damage, and the body tightens the muscle for protection, as it is stronger at the less long length so it’s easier to keep it safe there. Or over aggressive stretching, like in MMA or dance, or any sport. Or, maybe you usually pick stuff up from the floor, like shoes to put on, bit of litter on the floor, kids toys etc by using one side/leg more, and it’s sort of good morning/stiff ish knees deadlift exercise, on one side, and it’s the non tight side as it’s lengthened and strengthened regularly. So there wasn’t actually much ‘bad’ happening to the shortened side, you just don’t work it in the longer range, and when you start doing two leg exercises, like squats and deadlift, that imbalance can cause strain of the shorter hamstring and even pelvic rotation to the opposite side, which can lead to knee and back problems. So pay attention to yourself, and treat yourself nice.