Medication lose its effect

Hi
im 30 years old and have been diagnosed with add. My question is in regards to the medication lossing its effectiveness usely after 4 days of taking it .I first started on regular ritalin ( 10 mg 3 times a day ) this made me have a lot of ups and downs ( highs and lows ) so we changed to ritalin slow release , but after a about a week it seem to stop working or only be help full for a few hours . Then we changed to concerta er (36 mg once daily in the morning ) This worked well for about a week then it seemed not to work aswell  . After about month been on concerta 36mg my doctor changed to concerta 54 mg and for the first 4 days was great , I felt extremely carm and was able to concentrate for very long periods of time. It has now been a week and i can feel that the medication does not seem to working as well as it did and for not as long . My concentration seems to come in waves , for the first hour that i take the medication i'm ok then for 2 hours I am unable to focus on anything etc.
Just wondering why this is , I have asked my chemist be he doesn't seem to know
Any help would be great
Kind regards John