CO washing - what did i do wrong?

This is just me - and maybe I'm an anomaly. But I found COing didn't work for my thin, fine hair until I diluted the conditioner significantly with water and then left it on for 20-30 minutes.

At this point I have it down to a science, diluted conditioner until it's very watery, massaged well into the scalp (to break up sebum) and left on for 20 minutes, no more flat, greasy roots.  🙂

I think you'd have to put it on your roots really. Just not the conditioning stage. Otherwise you treat it pretty much like a normal wash. If you don't, you're just wetting the grease and dirt which always just makes it kinda nasty!

My hair loves conditioner, so I use a strong conditioner for wash and condition. But most use a cheap, thin conditioner for the wash and then a better one on the lengths for the conditioning.

I use an el-cheap-o conditioner too, and never a heavy-duty one such as for damaged hair. I just use one for normal hair or once in a blue moon, a "clarifying" conditioner. The fewer potential heavy ingredients, such as oils, the better, at least for me.

Various Suaves are my go-to.

You need to put it on the roots and work it in just like you would with shampoo.
I really like the ultimate blends one for washing, but you might prefer something lighter. Alberto Balsam or supermarket own brands (most cheap conditioners are cone free)
I do think your mistake was not really working it in. It doesn't foam like shampoo but you can definitely feel it cleaning your hair

I've taken a long time to actually go for it, it was all the slight suds after i used it as conditioner the other day that made me think - of course this stuff can wash my hair!

I am going to attempt using it on the roots after another olaplex attempt!