calcaneal fracture

I have had a calcaneal fracture since 04/04/03.
This entire time I have remained immobile.
First in a cast for 12 weeks, now in a cam walker (non-weight bearing).
Is there any specific reason why the fracture wouldn’t be healing after 5-months.
I have had frequent MRI’s to determine the non-healing.
It also shows that the fracture is non-displaced (which I hear is good).Any suggestions would be helpful!! Thanks!