Hair may be to wet. Towel dry , using your hands blow dry the hair until 60 - 70% dry, then use a Brush.
Not getting the Oomph... use your hands near the roots, the brush just doesn't get close enough.
Not starting at the roots, place your round brush at the roots, roll down to the ends, then take it back up to the roots, concentrate your blow dry near the roots & mid strand, by then the ends should be close to being dry. Now roll the ends in the brush and finish drying.
Make sure you're using the right products, if you want volume apply products to the root area. Spray from a distance. Too close causes build up & weight.
Use the right brush. The bigger the brush the smoother the hair. Metal cores act like a curling iron. On course hair consider a boar bristle or nylon brush they tend to have more bristles which gives more tension.
Not letting the hair set. after round brushing allow the hair to cool down (set) before manipulating for a longer lasting curl.
Use the Concentration nozzle. Yeah, it came with the blow dryer, it give the heat focus and seal the cuticle for smoothness & shine.