Do your meds sometimes not feel like they work?

Im on 40mg methylphenidate hydrochloride and most of the time I take my meds and everythings fine. Standard side effects like increased heart rate and dry mouth ect but I can also get my work done, I can think clearly and I'm generally productive.

Other times, I'll take it and it feels like nothings happened? Like I'll try to read something and nothing will go in, or I'll focus on something but just zone out. I dont know why this happens and it feels like my day is wasted. I'll still have the side effects, just not the desired effects. Does this happen to anyone else?