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
James Baldwin
A Talk To Teachers
[...]
one of the paradoxes of education [is] that precisely at the point when you begin to develop a conscience, you must find yourself at war with your society. It is your responsibility to change society
[...]