I have a flat root problem too and have discovered a solution that works for me.
I have to share this because my curls turn out so good.
I have very fine 3a hair, cut just below the ears and layered all over. It is near impossible to get lift at the roots for me. No product helps and often products weigh it down further which makes it worse.
So the trick is in the way my hair dries.
I do everything upside down including washing, rinsing, blotting moisture out and putting in products. The key is to rake the product through with my fingers in the OPPOSITE direction to where my hair will fall including the hair on the sides of my head.
I rake in with my fingers a curl cream (Curl & Mold Activator by Yellow) and some sea spray (Sea Salt Texturising Spray by Toni & Guy). Then with the palm of my hand I push the curls towards my scalp and hold them but do not scrunch with my fingers. After doing this for about 10 minutes I stand up and just let my curls fall. I carefully reposition them by holding them from the ends but I DO NOT rake them back and I DO NOT touch the roots.
My hair feels weird as if it's standing on end. I AIR DRY them. My curls turn out far better than diffusing then. I usually do this while I'm sleeping. I DO NOT sleep on my hair. I push my hair upwards so it's not resting under my head but behind my head on the pillow.
In the morning my curls are defined and soft. I tilt my head upside down and use a diffuser on a low setting to loosen them. Then i spray in a bit of dry shampoo at the roots and carefully rub in with my fingers. I then apply a small amount of curl definer into the palm of my hand and smooth over my hair (Round Trip by Paul Mitchell). Again do not touch the roots and use this sparingly. I then use a sculpting cream (Texturising Glue by Toni & Guy) to define each curl. You can use this product at the roots but don't go over board.
This is the BEST product. It gives alot of hold without any crunch whatsoever and no wet look either and doesn't weigh my hair down. It leaves my curls looking natural as if there is hardly any product in it. I then finish off with a medium hold hairspray.
And voila, I'm done!
The key is not to use too much product. Less is more.