Did you dilute it with tap water or something like distilled water?

I dilute mine with Distilled Water. I think it was MamaP that brought this to our attention. I think it's important to use Distilled, as tap water could have some bacteria, that could affect the Downy. I always dilute my Downy, or I just buy the Non-Concentrated April Fresh or Clean Breeze versions. They are very thin, ready to be used, do not clog the dispenser and the scents are lighter!

I suppose it could even be as simple as messing up the ratio of the preservatives and emulsifiers in the original fabric softener to where they are not as effective.

I mix mine in a 51 oz Downy bottle. I add 1 cup Downy, then fill the bottle full of water which is a little over 5 cups water. Works great for me. I dont use distilled water, never thought about that and I have done this for years. (but I have well water).

There's not enough preservatives in the Downy to kill the bacteria in tap water that will grow in a couple weeks, than you will be rinsing your clothes in bacteria, always use distilled water.

I didn't use distilled water so I guess it could be bacteria or just breakdown of the downy. It still softens but I guess it has to go down the toilet.

Can you just use boiled water instead of distilled?

Also, I had FS thicken significantly without any water added, so I think it just has limited shelf life anyway.