Varying effect of meds

Does anyone else experience variation in what the effect of their meds feels like?

I've gradually increased my dose to 2 x 30 mg of slow release methylphenidat and have had a couple of days where I felt fantastic. Very focused but also calm and balanced. I had this very strong feeling of "really being myself" as in being able to use my full potential as a person.

Then I have some days where I barely notice anything and today I've mostly just felt jittery and a bit anxious. I can feel the stimulating effect on my body (higher heart rate) but without the mentally calming effect.