Hey... I've been growing for some years and have the same problem.
Advanced Nutrients... Pretty much the whole line... Botanicare CalMag Plus... Hydroton... Recirculating bubble buckets, but the last year has been flood and drain... was 600 watt mh veg and 600 watt hps flower... now t5 veg and 1000 watt double enders for flower. ..
I've been thinking I use calmag too long in flower (all but the last week)
Maybe it's too much nitrogen (from the calmag)
Maybe not enough Phosporous? I heard increasing that can increase density... (or was that k (Potassium)?)
I've tried different humidities..
I've tried more lights...
I runn PPM at least 1000 in flower
Always adjusting ph from 5.5 to 6.1
They've NEVER been dense.. always airy. I've ran tons of strains also.. some several times from clones...
Please help.
I'm thinking that I'm going to try to start adding gravity because I heard it hardens them up.