Try adding some alkali to the blix. If you get the red color back and can smell a slight odor of ammonia, then the pH of the blix has become very much too acidic. I don't know how, unless you have somehow gotten stop bath or the equivalent into the blix.
If the addition of base does not turn the blix red, then look for a precipitate at the bottom of the tank. If that is present, then the blix has become very bad due to extreme acid or base contamination. Base can lead to ferric and ferrous hydroxide forming and acid can destroy the hypo, but it would take a lot of acid or base for this to happen.
Is the water too hard? If so, a lot of calcium and other salts can interfere with the blix, but I am not familiar with the results or which specific ions to suggest you look for. (see below though about copper and brass)
Are there any copper or brass fittings used in error connecting up your tank? If so, they will exhaust the blix and you will eventually spring a leak somewhere due to destruction of the fitting.
Last but not least. Did one of your tanks perhaps have silver accumulate in them and it somehow got used as the blix tank (like mixing a B&W or C41 fix tank for the current blix tank). The silver residue on a fix tank wall can rapidly exhaust the blix.