Under that kind of lighting, you should have no problem at all getting dense buds. Are your plants growing healthy from start to finish? I grow under a single 1KW and the only way I get airy buds is when the plants are really stressed. I fought a 4 month battle with root rot and it made sure all my buds ended up weighing nothing come harvest time. Very frustrating. But that is the only thing I can think of other than genetics. Are they from a breeder or bag seeds?