Using a baking soda with hard water makes this worse. A site I came across explained that "Soap is less effective in hard water because its reacts to form the calcium or magnesium salt of the organic acid of the soap. These salts are insoluble and form grayish soap scum," (sound familiar?) "but no cleansing lather. Detergents, on the other hand, lather in both hard and soft water. Calcium and magnesium salts of the detergent's organic acids form, but these salts are soluble in water." http://chemistry.about.com/cs/howthingswork/a/aa082403a.htm Baking soda and water form a very mild soap when mixed with the oils in your hair, shampoo is not soap; it is a detergent which is why it works in all kinds of water. But you can make baking soda work!
The solution is fairly easy. I use distilled water for my baking soda mix, which costs less than a dollar a gallon at the grocery store, just make sure it is distilled not drinking water other wise you will have the same problem. I mix about 24 oz distilled water with 2 Tbs baking soda, but use whatever has worked for you. The solution will feel slippery which I never could achieve using my hard water. I also apply before I get my hair wet, to reduce hard water contact, but I rinse in hard water and it works doesn't interfere. I have also heard of some success with boiling water, though I couldn't get it to work. http://www.how-to-boost-your-immune-system.com/no-shampoo-method-with-hard-water.html I was getting really frustrated with the no shampoo method until I tried this, it makes my hair feel very clean and lovely.
I also use lime juice as a conditioner, 2 limes to 24oz (you can use hard water here). Partly because of my hard water. If you have hard water stains in your shower dipping fixtures and such in citrus juices helps break up the mineral build-up, the principle seems to apply to mineral build up in hair as well. I haven't tried lemon juice but I imagine it would also work.
I hope this helps those struggling with the no shampoo method due to hard water, it has made me quit before but now that I understand how to prevent mineral build-up I see no reason to go back!