I made stock last Sunday, which I intended to use as the base for homemade tonkotsu (pork broth) ramen.  Accordingly, I used both chicken and pork (roughly equal amounts of chicken backs and pork hocks).  I also added several cloves of garlic, a large leek, celery, carrots, and small knob of ginger.    I boiled the meat/bones briefly before straining and starting over with cold water (for clarity of finished stock), added the other ingredients, and brought to a boil.  I then reduced to a simmer and skimmed the very small amount of scum from the surface.  The stock simmered for about 8 hours before I strained it through a cloth.  The resulting stock is perfectly clear, with an excellent, thick mouthfeel and gelled nicely in the refrigerator.  However, the stock also has very little flavor-- even copious amounts of salt don't help much.  Since my stock is by all other measures a success, I'm puzzled as to why it's so flavorless.    Where did I go wrong? 
I've always made stock using meaty bones with salt added. I have borderline low BP and can use the sodium. But I've heard many people making perfectly good stock without salt so I don't think that in itself is the problem.  If others agree your stock lacks flavour, it won't be due to your method. I'd say it's more likely your meat. Remember that chickens are slaughtered at a very young age - a matter of weeks - so the chicken backs would be from young chicken. Older larger chickens would be left whole for roasting. The same goes for pork hocks. A grown pig is a very large animal and you'd be able to judge from the size of hocks you used, the pig they came from wasn't that old either. Most pork we eat comes from pigs between 5-10 months of age - no longer babies but comparable to roughly a young teenager.  Old hens and parts from fully mature pigs was what was traditionally used to make stews and soups. The meat was too tough to fry or roast but was very flavourful for slow simmering. Think of the difference in flavour between veal (calf) and beef. 