community of dyers

Urea will go bad, eventually, when dissolved in water. It's natural for it to turn to ammonia. That's why diaper pails quickly develop a nasty ammonia smell. (Although the urea we use is made from natural gas, not urine, it is the same chemical, susceptible to the same chemical reactions.)

Would it be possible for you to get a small refrigerator to store your chemical water in? That would make a big difference. I like having a refrigerator for dyes, because the mixed-up dyes themselves stay good in there for weeks or even months, as long as their pH is balanced (with no soda ash and no ammonia smell). If not, perhaps you should resign yourself to mixing up fresh chemical water every week, not adding the urea so far in advance. Not a very nice answer, I know. Urea dissolves very quickly, so at least it doesn't take much time to add it fresh.

Whay is this happening to you? It's probably just bad luck. How quickly the urea to ammonia reaction proceeds may vary with the temperature of your house (though 70°F certainly seems reasonable), what other chemicals are naturally found in your water supply, and which microbes happen to be around. It's hard to control which microbes you have in your environment; it's considered to be a bad thing, for example, to make home-made vinegar in the same kitchen in which you also brew beer or wine, because a little bit of the vinegar-making bacteria will quickly spoil beer or wine.

A couple of years ago, Donata Pfisterer posted on the DyersLIST mailing list she had observed that chemical water goes bad more quickly when it is stored in plastic milk jugs than when it is stored in plastic detergent buckets, which are a different kind of plastic and are much thicker and heavier. Does it make a difference in your case if you use a different container to store your chemical water? What are you using for storage now?

Another option, a simpler one, is to give up the use of urea altogether. Urea helps to keep the dye moist on the fabric, long enough for it to react, but it is not always necessary for that, depending on your humidity and what exactly you are doing. In a dry climate, you can wrap tie-dyed items in plastic, instead. It also slightly increases the solubility of some dyes, but not more than maybe 20%, which is not a very big effect. Several tie-dyers have said that they have given up urea with no ill effects. It's worth considering, since it's been giving you so much trouble.

I would discard the chemical water once it begins to smell of ammonia, not use it.