Another possible problem - the dosage may not be right for her. Although if others even older are on the same dose - frankly, I think MidwestMom is right, you need to keep talking to the doctor who prescribed this and maybe ask for further tests. When we first began to medicate our children (those who are on medications) it took some times to get the medication right. Plus, as they have got older and their needs have changed we've needed to change their medication as well.
See what she is like on the weekend, see if YOU can see the improvement during the day. A few things to watch for - it's early days for her too, she is still trying to learn self-control with a mind that is now more unpredictable than it was. It will take time for her to adjust, especially to change in how she copes, as medications begin to work, or begin to wear off. Plus, there CAN be another problem called rebound. This is where, in some cases, the symptoms suddenly become much worse as the medications are wearing off - worse than if the child weren't medicated at all. Only some kids have this problem and with different medications. My son had rebound on ritalin but not on dex. A friend had rebound on dex but not ono ritalin.
Also, keep a diary. The computer is good for that - each day make notes (short ones if you don't want to do too much). Write down any good things, any bad things and anything interesting. We used to use a record like this and sent it to school each day for the teacher to write in as well. It made it easier for us to know how the teacher was managing, and what sort of problem they had at school. It also helped us see connections between bad behaviour at home and problems at school which we might not have known about, without the diary.
She is only six years old. You've been through some terrible times but it should soon begin to improve. You've made a lot of progress with finding someone who believes in you and wants to help your child. Often parents struggle for a lot longer, being told it's their fault. It isn't.
I may have some other suggestions for you to consider, later on. I don't want to overwhelm you right now.