If you've ever tried to memorize something meaningful like a poem, a passage of scripture, or a speech, this has probably happened to you. You learn it, and it feels solid. Then life gets busy, you miss a few days of review, and three weeks later you're surprised by how much has slipped.
The instinct is to blame yourself. Bad memory. Not disciplined enough. But that's the wrong diagnosis.
What's actually happening is that your brain is doing exactly what it's supposed to do. Memory is a living system that prunes what it isn't using. If you learned something once and never retrieved it under increasing time pressure, your brain reasonably concluded it wasn't worth maintaining. That's actually pretty efficient.
But the problem is that most memorization tools work like memory is instead, a filing cabinet. They help you put the text in, but you have to do the work of keeping it alive.