My hair isn't particularly fine, but it does go annoyingly flat. I use root volume spray and blast it with the hairdryer, and that does help. Another trick I use sometimes is to kind of 'kink' my roots with my GHD's....and that can work really well sometimes...(but to be honest a lot of the time my hair seems to have a mind of it's own, as I can use exactly the same techniques on two consecutive days and have completely different results!!)
Try this for volume at the roots, oomph powder. Just put some on your fingers or directly onto the roots then kind of massage in and plump up at the same time. If you feel your hair flattenning during the day, just rub your fingers into the roots again to give lift. There are some tutorials on youtube of how to use it.
And, if it doesn't work- well, it was only £4.19 (and is on 3 for 2 right now).....
Why not forget about giving volume to the root and concentrating on the ends instead (not explaining this well!). So leave it smooth on top but use rollers or curling tongs to style/give emphasis to the rest of your hair.