I have no idea of what your soil is like, so this may help or it might be totally useless infomation.
I did somthing similar on a project in California and had trouble keeping the stones in place. A quick fix was to use expanding foam and liquid nails to hole them in place, but it got to be to expesive. Then I saw a program on TLC wich had a contractor backfilling with concrete to lock them in place. I mixed up a few sacks and it seemed to work. My wall was only about 2 feet tall at the highest, but fairly long to creat a flower bed using native sandstone taken from a seasonal creek bed.
I love the look of dry-stacked stone and it looks like your doing a great job!!