Why does Vyvanse feel different every time I take it?

Some days it feels like too much and other days it feels like nothing, all while being on the same dose. I know Vyvanse has to be converted in the body for it to work and I’m wondering if my body has a hard time metabolizing it. I love Vyvanse when it works, but I’m starting to hate the inconsistency.

Has anyone else noticed this? What could be causing this? Does this mean a different medication might work better for me? Thanks!!

Edit: I’m taking the generic and I’m in the US