My middle DD was like this....seriously it did my head in! I felt like the constant nagging mother, forever on her back about something, but if I wasn't telling her to do something then I knew it wouldn't be done. As for losing stuff, well, that was a constant battle. DD#1 was super responsible and never lost anything, #2 well, it was a constant checking of the school bag when picking her up, making her go back to the classroom or the playground and find the lost item and I didn't let it drop, I kept on about it, she eventually got the message. DD#2 did have a lot of assessments done when she was around 10, and a lot of counselling due to other issues, but they found that there was nothing wrong, she was just very absent minded, and that I had to give her information in short bursts, rather than a list of things, because she couldn't cope with lists, she would only remember the top couple. The counselling sessions helped me too, and it is amazing how much of it I am using now with #3.
#3 is such a handful, but she certainly knows how to push everyones buttons, and play the baby of the family. I am trying to get her to take more responsibility for her belongings and so far, so good, but she is also getting more chores, which is making her feel more grown up and like she is an active member of the family.