First off: Congratulations on getting diagnosed. It's a huge step and 18 is a really nice timing. Untreated ADHD in adults can be really crippling, because all the supporting structures like school and family break away and suddenly you realize that managing your own time schedule and house hold is really fucking hard. So good job 👍

Second: You took 30 mg of Elvanse/LDX for the first time and you felt nothing? That's... impressive, honestly. Methylphenidate Non Responder are fairly known, but usually that amount of dexamphetamine does something to the human body. From your age I'd guess you are not a regular user of recreational amphetamines and have built a massive tolerance...

I also started with 30 mg Elvanse and was pretty seriously zoned in, deep focus, elevated mood, highly awake, loss of appetite and a very revelatory feeling, that I could finally tackle all the bullshit that I had avoided, delayed, ignored and forgotten. For me, the 30 mg were honestly too much. Over the last year I've built a tolerance and am now taking 40-70mg daily, depending on the challenges of the day.

Still I wouldn't worry. No effect also means no bad side effects. Adjusting the dosage is entirely normal in the beginning and something your psychiatrist should do anyway. The therapeutic dosage goes up to 70 mg and there are edge cases with even higher doses. There is also methylphenidate (Ritalin, MPH, etc) and Atomotexetin. But for now, just stick with what you got.

Dealing with ADHD is a marathon, not a sprint. You've just started, so take the days as they come and trust your feelings above all else. If it works, you'll know. And if not keep at it.

Also check out /c/neurodiverse, there are a lot of good posts and ADHD experiences shared.