Help! How do I get my 12 year old stepdaughter to understand the importance of good feminine hygiene?

My 12year old step daughter got her first period when she was 11years old. She has everything she could need available to her - I know what it is like to be young and embarrassed about getting my period, so I tried to relate to her and give her all the information she might need about what was happening, what to do, what pads were and how to use them and even how often. But I continue to find old blood soaked underwear in her closet. I have tried on several occasions to figure out what she isn't understanding or why she doesn't care about her personal hygiene in general. I've tried to explain the social reasons for maintaining good personal hygiene and health risks of not maintaining good personal hygiene in general and especially her feminine hygiene. Nothing seems to get through to her. Her grandmother has probably tried more times than me- we just don't know what else we can do. Anyone have any thoughts or advice. We would appreciate anything that might help. Thanks moms!