A wash aid is indispensable for getting well-washed prints. There are lots of threads on this and lots of charts/tests/explanations already that you can search for, so I won't bother going into that here. It is difficult to get really well-washed prints with modern papers without a wash aid
The oxidation of the sodium sulfite in the wash aid limits its lifespan. It won't do its job if it's oxidised. Working solutions go bad in a few hours. Stock solutions can go bad as well, but have longer lifespan . I mix my wash aid fresh for each session (1 Tbsp sodium sulfite plus a pinch of metabisulfite per liter) and make sure I don't exceed capacity (check the Kodak HCA package for capacity if you're not sure) in order to be assured of fresh solutions.
With powdered bulk chemicals, it is not recommended to mix smaller portions of the contents. However, HCA is almost completely sodium sulfite, so mixing smaller portions of this is less problematic. I have done this in the past with HCA and it seemed to work well. This is one possible solution to the aging problem if you don't want to mix one-shot from scratch.
There's no reason to use too-old, exhausted wash aid, however, so figure out a way to deal with the constraints some way. Mix one-shot from scratch like I do, or try to do enough prints to use up the stock solutions, or try mixing just bits of the HCA at a time, or just toss the stock when it gets too old and deal with the small financial loss. All are better than incompletely washed prints.
As I mentioned earlier, I give fix one and then wash my prints for an hour, dry them, and collect the keepers for a toning session. Toning sessions consist of a soak, fix 2, toner, wash aid, rinse, wash, and stabilizer. I can save up enough prints to use up quite a bit of wash aid in a couple of days this way. Maybe it could work for you as well. I try to keep the time after the first wash/dry and before toning to a minimum, but I imagine that you could go as long as a month or so without degradation as long as you wash a minimum of on hour the first time.
Hope this helps,
Doremus
www.DoremusScudder.com