Talk to her doctor about your concerns. If conservative care hasn't helped in four to six weeks, then surgery may be needed. The fact that she's losing function is a red flag.
If it hurts to move, the average older adult will stop moving. Motion, function, and strength are quickly lost and can be hard to get back. Any further treatment may be less successful because inactivity and decreased mobility has led to deconditioning.
Your mother may be a good candidate for a procedure called vertebroplasty. A long thin needle is inserted through the spine. A cement or "superglue" is injected through the needle into the bone. It fills in all the cracks of the fracture and helps reinforce the vertebra.
In a similar operation called a kyphoplasty, a tiny balloon is blown up inside the bone to help restore its size and shape. Then the glue is inserted inside the balloon. When the glue dries, the bone is back to its original shape and strength.
Patients have better long-term results with improved function when they go back to the physical therapist after the vertebroplasty. Patient education is a key feature to maintaining independence.
The family can work with the doctor, nurse, and physical therapist to help your mother understand the importance of doing the exercises. There may be a specific reason why she isn't doing them (e.g., forgets, confused, tired). See if you can't find out why she's noncompliant. Closer supervision by the therapist may be needed at first. Friendly reminder phone calls from the family may help, too.