I boiled all the bones from a roe deer for stock and got about four litres all of which was solid when refrigerated. I took half of it and left it in a pan in the bottom over of an Aga for two days to gently evaporate and become more dense. The result turns out to be a great colour and smell, but will it doesn't solidify. Why? The remainder, though less concentrated will not come out of the jug even if tipped upside down.
Sounds like the temp of the Aga was a bit low. From the GMIA Gelatin Manual (page 11):
As a dry powder, gelatin is very stable, and can be stored in air tight containers for years with no loss in quality. Gelatin in solution, or soaking in water, should be left in this state only if kept very cold, or hot enough to destroy or inhibit bacterial growth. The nature of the organisms which grow in gelatin solutions and gels depends upon a number of factors. The pH has a most important influence. At pH values less than 4, bacterial growth is suppressed, while yeasts and molds grow abundantly. Above pH 5, proteolytic bacteria can become active.
There are two things I can suggest: 1) Try breaking the bones up a bit, likely with a tenderizer or a hammer, and 2) the cartilage is where most of the gelatin comes from when making stock. Hope this helps.
I think it was the salt content of the reduction? I guess salt interferes with gelatinisation of starch. I googled it and this is the best I can do: