Do you have layers at the crown? Get your hair dresser to create more layers so that when you style it, it won't be weighed down.
I get my volume with two ways:
1. Apply a thickening paste like Got 2 Be's Fatten Up, blow dry hair with a large round brush in sections, then immediately put sections of the crown with velcro rollers. Let it set for about 15 mins, and then spray lightly with hair spray. Style hair with fingers. I get full bouncy hair.
2. Spray sections of hair with hairspray and backcomb the roots. Smooth it out and cover it with top layer of hair. Backcombing is a little more work but it does not go flat easily.