Arno,
Take a look at your pelvis when you forward bend. As in Paschimottanasana, the pelvis should be moving as you come forward. Place your hands on your hips and start to come forward. Stop coming forward when your pelvis stops. If you continue, you are using your back, not your hamstrings to pull you forward. With some of my students, I take a yardstick and measure their progress. Place the yardstick right at the knees and then out beyond the feet and measure when you come into forward fold. That is your starting point.
While laying on your back with knees bent and soles of feet on mat, extend one leg up toward ceiling. Again, check your pelvis to see if it is tilted. Look at your extended leg. If your hams are tight, knee will be bent and leg will not be perpendicular to ceiling.
Standing wide leg forward bend is a more gentle hamstring stretch for those with tights hams. And remember, if you want to lengthen, hold the pose longer and come out of the pose very slowly, concentrating on the breath as you come out. Muscles have memory and if you are constantly going from pose to pose quickly, the muscles will strengthen, but stay contracted and shortened. Dynamic repetitions coupled with holding the pose and then a slow controlled release will help lengthen.
One more thing. How far down can you come in Uttansana? Are your knees bent in order to touch your toes? If so, get a block and place it in front of you. Stand in Tadasana and place hands on hips. Begin to spill forward and when the hips stop moving, so do you. Keep knees straight (not hyperextension) and place hands on blocks. I know many teachers tell their students who cannot reach their toes in the above pose, to bend their knees. But if you are working toward lengthening the hamstrings you won’t be lengthening them if the knees are bent. Legs have to be straight.
Hips play a huge role. Pay attention to how they move. Measure your progress. You’ll be surprised how quickly you progress.
Supta Padangusthasana with the leg straight up to start and the foot flexed is a good stretch. As you progress and are able to keep the hips on the mat, you can release the leg out to the side. With the leg that is out to the side, bring the opposite hips toward the lower ribs which will open the hip of the extended leg side. My students have found this to be most effective. You can also hold this position in a more yin style.
Best of luck!