Need advice

I was diagnosed adhd at 36 years old, doctor proscribed me to amphetamine salts ER 15mg capsules. My first dose I took in the a.m and felt great the entire day. Could focus, get things done without hesitation. Everyday after that it seems as it doesn’t work anymore. Like I haven’t taken anything and it’s been a week since my first dose. The pill is baby blue/white and the numbers match what’s on the bottle. Why did it only work the first dose?. I don’t drink acidic drinks and I eat a high protein diet. Brand name is granules pharma