Infection (presumably docs have ruled this out)
Diabetes (I'm guessing no)
Hyperthyroidism (hmm maybe?)
Repeated injury to the site (possible?)
Poor nutrition (doesn't sound like it)
Anemia (you already discounted this)
Phosphorus, copper, calcium, vitamin d, magnesium etc (nope)
Complex break that can't be expected to heal without worryi etc (docs would have said before)
Medications (some anti inflammatory etc can cause issues)
That's about all I can think off to be honest. So meds or hyperthyroidism are the only 2 we can't really discount at the moment. The thyroid is a very difficult thing to check though, because levels carry so much from person to person is hard to know what is your correct value.
Meds should be easy to work through...
If you really want some miracle cures to look into (do your own research)
Comfrey (aka knitbone)
L-Arginine & L-Ornithine
Personally if I could only recommend one supplement it would be oxygen elements or similar product. Not sure if it would help with bones, but it can help fight off cold s, negate hangovers all together etc. My mate Hates this sort of stuff, but he had a cold before his stag do.. fixed overnight.. he dam near died of alcohol poisoning one night but woke up fresh as a daisy... This hospital always comments on my high oxygen levels as a good thing.. bones need good circulation to heal, cos circulation improves oxygen levels.. so do these drops.. I don't often recommend stuff like this because people are usually in 2 camps. Either fierce critic of hopeless optimist, all I will say is that this is the one supplement that has actually impressed me. Combine with Vicks first defence and you are immune to colds end off... Venturing off topic now
One further idea for you is get yourself on a qxci machine. I had a couple of good experiences with them too... Might help explain what is going on. Don't wanna come across as an adult therapy wacko so will stop now. Well let you do some reading.
Failing that I'm out of ideas in afraid