"The lid of a jar can be stuck, and your car can get stuck in traffic; either way, the thing that's stuck isn't going anywhere. You can also use stuck when you can't figure out what to do"- vocab.com
It is common for rider's to feel " stuck " in what they are working on or stuck in their habits.
What about when our horses feel stuck? They won't go forward, they won't bend one or both direction, and they fall in on the circle. Do they need time off, a chiropractor, massage, or injections? Most often, these can be recognizable signs of improper muscle use. Horses just like people compensate for any imbalance, pain or fatigue. There will always be an underlining issue of a horses change in behavior or movement. You need to find the root cause and clear any medical issues with your Vet.
Riding and training without understanding of the movement, has no meaning or benefit to the horse. Pushing a horse to move forward with spurs and a whip will not create forward. This mentality creates more resistance and can eventually impair the horse physically and emotionally. Pulling the horses head and neck in while going round and round will not create bend, it will cause unnecessary muscle strain and create abnormal gait movement.
Falling in on the circle is an inverted use of back muscles. Riding a shoulder in without flexion is a great exercise to test this theory. If the horse cannot cross the hind legs, they are usually using their back muscles improperly. Start over at the walk...leg yield, shoulder fore, shoulder in, half pass and walk pirouette will help supple and release the contracted muscles. It's like physical therapy for your horse. We need to always be thinking of the horses and what they are communicating to us. They allow us to not only ride them but also depend on them for our emotional support.
"All good riding is based on love for the horse" -Charles De Kunffy