What should I do if my back problem does not improve after physiotherapy?

There are 3 main reasons why there is no improvement after physiotherapy treatment for back pain :

1. incorrect or undiagnosed pathology, e.g. the treatment may concentrate on degenerative discogenic pain but unknown to both the therapist and patient; the pain may be caused by something else like a fracture, infection, cancer in the back or from somewhere else like kidney stones, uterine/ovarian disease (further investigation by your doctor is required).
2. inadequate treatment, e.g. less than 5 sessions in total, each session less than half an hour; no weekly sessions for at least a month; no daily self-stretching/core-strengthening exercises at home after your physiotherapy, etc.
3. irreversible worsening of your degenerative disc, leading to either disc rupture and/or unstable spinal bones (spondylolisthesis) with severe nerve compression - may either worsen after your first earlier MRI (hence must repeat MRI again to confirm) or may already be revealed by your MRI to be serious but patient is not keen for surgery (unlikely physiotherapy will help without surgery).

You should get assessed by a spine specialist, who will likely get an MRI done for you.