The Ammonium Iron Citrate does go off. It seems some microbes find it tasty - it must contain enough trace impurities to sustain life.

You might want to try the "New Cyanotype" - it uses Ammonium Iron Oxalate - and is reputed to be more stable. It is, however, a bit of a PITA to make when compared to plain-ole-cyanotype. Contrast can be increased with P. Dichromate.