Over the past couple of weeks, my baking-soda-washed hair, while still looking pretty good, had developed a yucky, coarse texture, to the point where it actually became…sticky. Now, sticky is obviously unacceptable, but I wasn't willing to go back to frizzy, either. So I Googled, "wash hair baking soda sticky disgusting" and got a possible answer: It doesn't work well if you have hard water.
Well, did we have hard water? I had no idea. So I Googled, "How can you tell if you have hard water" and found a test. You fill a water bottle half full, put in some dish soap, shake it up, and pour it out. If the suds pour out the bottle, your water is soft. If they stay in, it's hard. (I did not need to perform this test, by the way. I was astounded by the information that suds ever would pour out of a bottle.) Hard water it was.
Apparently the solution is to boil the water before using it. So I decided to get an old water bottle, put in the baking soda, and fill it with water boiled in the teakettle. Here's the point in the adventure where I should have Googled, "What will happen if you pour baking soda and boiling water into an old water bottle and then absently put on the lid and close the spout," but perhaps I'll tell you that story another day.
Anyway, I've washed my hair with the new stuff twice now, and it does feel a lot better while still maintaining its non-frizzy properties. So there's a bit of troubleshooting advice for you if you're thinking of trying this.