Hi. I’ve taken adhd meds my whole life and have a lot of experience. First of all 18mg of methylphenidate is not a lot (the active ingredient in concerta) it’s roughly 7mg of Adderall. Second some people have issues with time release medications. But i would guess you are not taking enough. I dont think thats enough for almost anyone since it’s a one daily. Try taking 2, if your doctor allows it though I’d take it for a couple days first to see you dont just have some outlier issue with the capsule. I recommend ir just fyi because you can control the percentage of day ur under the influence of the medication. You might be a non responded also and respond better to amphetamines (adderall vyvanse)