I am not an expert. I understand it is a controlled medication and there is a phenomenon of the body getting used to it, the sensitivity decreases with time. So, at the very beginning the body, or rather mind, would react sharply on a small dose and then the dose should be increased relatively fast to a suitable plato, where it is optimally effective. The next significant increase is years after, when the body weight increases as well.
My DC is taking Concerta for 12 years since the age of 7 without any negatives, and the effect is visible to the naked eye in terms of focus and behaviour. She initially started on Ritalin (methylphenidate fast release) and then moved to slow release Concerta, which avoids the roller-coaster during the day. I don't know about Medikinet, I understand it is methyphenidate all the same.
What I really want to say is that you seemed so worried and unconvinced about the medication, that there might be a 'negative placebo' effect, you might be poised to look for negatives and to interpret everything this way. Maybe you are not, but just consider this.
Maybe when your DC takes 20 mg the plato will be reached and you will start seeing an obvious difference.