I tuk concerta 18mg 1st time, it worked then I stopped it. I tuk it 2nd time 18mg but this time it didn't work so should I take ritalin (methylphenidate) or concerta rx.

Stimulant. See your physician. If add medicine does not work, it may mean need to recheck diagnosis, may need a higher dose or a different medication. Changes in stimulant medication should be under physician supervision.
Medications. Please stop treating yourself with amphetamines or other stimulants unless a physician is supervising this treatment. It sounds like drug abuse as you describe your use of this dangerous medication.
You are not abusing. I assume you are under psychiatric care. You are on Risperdal which you should stay on unless your psychiatrist says otherwise. Risperdl can blunt the effect of either Ritalin or concerta. They are actually both methylphenidate. I am gig to assume that you are of a normal weight for your age. If do, then the answer is to raise the dose of Concerta under a psychiatrist's to right dose for you.
Concerta. Talk to your doctor first, either adjusting the dose to 27mg. or even 36mg., or switching to Ritalin (methylphenidate).