Could be the distance of your light to the plant,could be the strain,could be your feeding schedule,could be your enviorment,could be your nutes...(see what i'm getting at)..what kind of water are you using? my friend had big problems with his buds being "airy" because he was using his well water(switched to fresh rain water and bam better results)...what i'm getting at is everyone gets caught up on the light..yeah thats important, but your temps,nutes,air flow, play major factors.. also i use cfls for side lighting and that made a huge difference for me....i knew someone else who had no fans in his veg area and he wasn't growing a strong plant because he never had any wind on the plants for stem strength.. so he grew a weak plant and when you grow a weak plant you get weak airy buds..so i would re-evalueate your grow area Temps, Humidity.. reflection,cfl side lighting,your grow medium,make sure your mixing your nutes right,i recomend bc nutes...i have had night and day difference with that product over any.. fox farm all of them..it's real quality stuff...do more grows keep trying...it took me about 4 or 5 grows to really understand what was going on and with each grow since my yields and density in my buds have improved dramaticaly..i will say the cfl side light 6-42 watt cfl's for flowering made the biggest difference for me..with the 250 watt hps and the cfl's i am almost copying a 400 watt light...