I used the PF commercial mix of DiXactol for the past several years until recently switching to Pyrocat. During that time I never had any fail, even when the A bottle was getting close to a year old and was pretty severely discolored. I question how the developer could "die" yet still develop film adequately, if without stain. I recall from the instructions that if an acid fixer is used the spent developer should be poured back in for a couple of minutes after fixing to enhance the stain. I realize that the OP states he uses alkaline fixer, but perhaps something in the fixer interfered with the staining?