I’m on 40mg of Ritalin a day and I’m having that problem. Originally I was on 20mg (My doctor had me on 5mg, RIDICULOUSLY, and I had to take 4 to feel any effects at all, so 20mg) and then my doctor tried me on Concerta & Adderral when I said it wasn’t enough – those things didn’t help at all and only had bad side effects, so i got back on the ritalin, but now a 20mg pill doesn’t do anything except make me moody. 40mg works for 1-3 hours but then abruptly stop. I hear 60mg is the limit but what if i just keep building a tolderance? This is the only med I’ve found that works! I’m petrified of it not working anymore some day. I just got my diagnosis this summer and I don’t even take the meds regularly (esp since i have to take 2, than another one a few hours later to get more than 2 hours of productivity) but I don’t know how I went 24 years without using them. No wonder i dropped out of HS! 
This was unproductive, but yes, i feel you! I also notice no amount of meds help if I haven’t slept well. I have a toddler and a possible seizure disorder though, so that is the norm.