I don't know how I missed this, sorry!!!
If you are only shampooing every month, then maybe you should consider using a sulfate shampoo, just to be sure that all of the buildup has been removed. Audrey Davis-Sivasothy's site has a page
about which sulfates are harsher and picking the right shampoo. NC also has a page
about what ingredients different detergents can remove.
I noticed the Cantu spray you are using contains a polyquat. Polyquats can also cause build up in between washes, here is a page
about that. I tend to think the more film formers (silicones, polyquats, carbomers, etc.) one uses routinely, the greater the need for a weekly shampooing and deep conditioning regimen.
Last, if you have hard water, maybe you need a chelating
shampoo instead of a clarifying shampoo. HTH