My guess is that if you look at the screen you will only be able to see 10 edit text boxes and the edit texts with index 10+ are off the screen and inside a list view so they do not actually exist to robotium, you will need to scroll before you can input text but then the indexs will be off so you will have to compensate. The alternative is to find the edit texts by a different means like id or a tag or to find them as a child of the listview row that they are in.
If you can confirm it is because its in a list view i can find some code to help you.