That's interesting. Well most dojo stretching is for about 10 seconds each, and a physical therapist told me that a stretch that's less than 20 seconds is a worthless stretch.
The physical therapist is correct, and I'd go so far as to say 30 seconds minimum.
You don't build flexibility by stretching in the dojo. The dojo is for learning new material and correcting errors in old material. It's for learning, not training. Stretches at the dojo are to warm up muscles and to teach stretching techniques.
Training is done on your own. If you're only stretching at the dojo, then you are absolutely failing at the "stretching properly, regularly and diligently" part. You need to be stretching every day. On your own. Work out a series of stretches that focus on your "problem" areas. Run through the series, at least 30 seconds in each stretch. Now go through it again. And again. Maybe one more time for today. Now do it again tomorrow.
Gradually increase both the time spent in each stretch and the number of repetitions of the series. Don't bounce; bouncing tears things. Focus on breathing and relaxing the muscles being stretched. You may be surprised what muscles are involved. Here's an example.
Lay flat on your back, arms at your sides or angled slightly out. Now raise your right leg, knee straight, until it's pointed at the ceiling. Without bending the knee, bring the foot across and touch the floor to your left. This rotates and stretches the waist. If it's easy to do, bring the foot further up, so the hip is flexed more than 90 degrees, and try to touch the floor further up your body.
Now do the same thing, but this time hold your head off the floor. You'll find it much more difficult, because lifting the head tightens the muscles in the abdomen, which will reduce your ability to twist the waist.
So relax as many muscles as possible during your stretches.
As with pretty much everything involving the human body, it's easier to develop if you start at a relatively young age.
