I have this problem too. The same routine/products work great for a week or so then crap out. I have to rotate products but my hair is still iffy on a daily basis.
I poo twice per week, at least one of those times with a mild sulfate poo, clarify monthly or so and by and large my products are CG-friendly (which should, in theory, mean less build-up). I don't see where I should have huge build-up problems. But that seems to be the most logical explanation, doesn't it? Continuous use of one product eventually results in certain ingredients building up too much, and also perhaps leaves your hair lacking other ingredients it needs.
My theory is that there is a magical state of hair balance: not too dry, not too conditioned, just enough hold, just enough curl boost, etc. But it's almost impossible to maintain that state. As soon as your hair starts to reach its zen, something changes (even just the weather) and the balance is thrown off kilter again. Some curlies know their hair very well, and can readily adjust their conditioner, etc. when needed. I don't have that hair sixth sense, so by the time I figure out what my hair needs, something changes again and around we go.
Those magical weeks might just represent a point in time when, for whatever reason, the stars have aligned and you have achieved the magic hair balance. Perhaps certain products have an ingredient or two your hair was craving at the time. I think laurabeth came to that conclusion in a thread about products that worked good at the start but then failed. She found a common ingredient or something in those products and was speculating that perhaps her hair needed a dose of that. I think...my memory is a bit hazy.
Also, just mho, but I think wavy hair is the most unpredictable hairtype, moreso that true curly hair or straight hair. I'm trying to accept that, as much as there's one perfect look I would love to get on a daily basis, it won't happen. I'm better off learning to accept my hair and find a nice look for it in all its states: nice and curly, flatter and straighter, messy or smooth, whatever.
Btw, I love your sig. I can relate!
2b-ish--embracing my waves this summer
Aussie Catch the Wave condish, L'Oreal NutriGloss condish; Nexxus Designing Texxtur Cream, Rusk Radical Creme, Pantene Curl Defining Mousse, Paul Mitchell Sculpting Foam; coconut oil for DTs