The Romanian deadlift and the good morning are often touted as the two best hamstring exercises. The problem is, they also heavily involve the lower back, and that's the muscle group that tends to "take over" for most lifters. That's especially true if your hamstrings are lagging behind. Your body will always try to shift the tension onto the stronger muscles involved in the exercise.
The Plate Trick
If you want to maximize hamstring stimulation during good mornings and RDLs, here's a trick based on one simple biomechanical principle: the muscle being stretched the most is the one being recruited the most.
With that in mind, a very simple way to get more hamstrings growth from these movements is to elevate the front of your feet by half an inch to one inch.
Simply put the front third of your feet on 5 pound plates or any surface about that height. This puts the hamstrings in a more stretched position, increasing their recruitment. You also shift the balance point to the back of the feet which enhances the recruitment of the posterior chain muscles.
You might notice that your range of motion will decrease compared to your regular good mornings and RDLs. That's fine. What you want is to feel maximum tension in the hamstrings; the actual range of motion is less important than getting that tension.
If you want to maximize hamstring recruitment, don't focus on how low you go. Focus on pushing the hips as far back as you can while feeling maximum tension in the hams. Keep your lower back arched. Use the lower back mostly as a fixator not a prime mover. Focus on having the hamstrings do the work.