My l5 vertebrae is not healing. why is this happening?

Spine fracture: Depending on the type- and location of the fracture, healing of a spine fracture can be compromised if there is too much stress on the bone. In the bottom of the spine, there are a lot of forced that keep wiggling the fracture back-and-forth, and prevent the bone-cells from bridging across. Often, surgical stabilization is required to allow the fracture gap to heal and make the pain better...

This is very vague: As to what you mean. If this is a condition called a pars fracture or spondylolysis this is typically from a congenital defect that will manifest itself in childhood but may not become symptomatic at all or at some point later in life. These typically do not heal but can be managed non op 90% of the time. If there is a compression or vertebral body fracture, they heal in about 3 months.