Ok, I'd like to say, when I first got my period I didn't understand that when your period came it stayed for a full five days and you need to wear protection for that entire time...not just when you're bleeding during those days. Especially because the first few times I only had a very short period, and it was very stop-start. It was only when I bled through my clothes (humiliatingly) at school one day when one of the other girls (very kindly might I add) explained it to me.

Perhaps you could also explain that the blood doesn't really always wash out completely and over time it will ruin her clothes? I also think that encouraging her to put them on to soak (when she does leak) and getting her to do that round of laundry through to foldingup and putting away will help her understand that there are real consequences.

I also suspect she might be in denial, or just doesn't want to mentally deal with it. She's pretty young still! Perhaps getting one of those puberty books and reading it through together and encouraging her to talk about her feelings might help.