Teachers! Have you had students who just could NOT loosen up their joints - preventing them from doing different asanas which kept them from getting the full benefits of yoga? 
Joints do not and should not loosen. As I’ve said, asana is about mobility and stability, not flexibility alone. Joints are comprised of connective tissue, specifically tendons and ligaments and while they do move they should not be stretched (much).
When muscle is stretched and it is done without mind, without intention, without purpose, it often goes right back where it was.
More to your question, since I cannot see my student’s home practice (except int he bodies they bring to practice) I can’t make any assumptions about what they can or can’t do. Doing requires ongoing practice. It does not often come from a class twice per week.
Thanks Athlete. Tree pose is an example of one of the poses I struggle with. I can balance myself on one leg and put the opposite leg or foot onto my tibia or knee but I cannot get my foot on my thigh. Backbends seem impossible. Im not pushing to do any type forward bends but they seem difficult. I will continue on regardless.
Yes, all the time. Don’t be discouraged if you find yourself struggling. With tree pose, start by either placing your foot below the knee, or cross the ankle over the knee. You can also come to a wall, with your fingertips touching, and gradually begin to wean yourself from the wall. The key to tree is being able to firmly press the heel into the thigh while keeping the bent knee moving out to the side. Keep practicing! There are always ways to modify poses that seem impossible. Eventually you will find the impossible transformed into, well, easy!
You’re welcome of course.
I would add the following…
Much like school, there are prerequisites and preparations. When you want to do Calculus you must be doing the things before Calculus that actually develop and guide you in that direction.
Carrying the analogy over to asana, in vrksasana (tree) if you are not able to place the foot atop the inner thigh then an asana practice focusing on hip opening will lead you there. An asana practice that doesn’t is still an asana practice. It is just not an asana practice specifically attending to the current needs of your physiology.
Bear in mind, it is perfectly fine to place the foot on the inner calf. However the foot should not be placed at the knee as the joint is not designed to take lateral pressure. The lateral collateral ligament is at risk.
Backbends are a similar issue. However with backbends you must realize we live in a forward bending world. We bend forward to do so many things AND we typically have patterns which deepen this groove - like sitting for long hours in a chair. When we are unwilling to change the behavior which has brought us to where we currently are, asana will still help but it will not hold back the flood waters of our choices.
Last night after reading everyones post I thought I would practice “vrksasana” the secret for me to get my foot up to the thigh and to keep it there is to remove my clothing. By doing this I able to dig the heel of my foot into my thigh and hold it there without the hinderance of clothing causing my foot to slide down. Im smiling just thinking about it. This may be totally incorrect, but it worked. I realize that I have been out of shape for years and its going to take time and patience to get into some of these poses. Im still very new at this and have so much to learn.
I have added a few hip opening exercies to my daily routine which will be of help.
ps. now i know why we dont place the foot on the knee in tree. It puts the collateral ligament at risk. I learn something new here daily!! love it!