Why will my dense, concentrated stock not solidify to jelly?
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.