Why are my buds airy?

My indica buds are dense but sativa/indica hybrids 60/40 are airy.
My indicas have hard buds but they have too much space between each one.
I know I under feed for 1st 6-7 weeks.Only fed 3 times,then I got a PPM meter and fed correctly.
Can under feeding cause airy buds?

Under feeding and light that is not intense enough can both contribute to airy buds. Light that is not intense enough can also cause the stretching between nodes you seem to be describing about the buds being not very close together.

Exactomundo!!! Give us some info on your lights. Sounds like some stretching occurred. Hard Buds are good! Once a plant has taken a shape, there is no need to woory about it. You have tight buds. Perhaps more intense or better light management from seedling to veg would produce the desired effect.

5w 400watt led and a few red 26watt CFL’s are feeding two plants in flower.
I feed my indicas at a constant 800PPM run off starting 2 weeks ago and they are looking much better,in fact they are still swelling and are within 1/2in of each other now.If my trich stay clear they may finally meet.
I let run off go back to base-line on sativa hybrid before I fed them.Basically only every two weeks they got fed.I did same thing with indicas until 2 weeks ago.Directions say once per week or more if they react,so every 5 days keeps it at 8-900PPM and they seem to like it.
I’m tempted to feed sativas but main cola and second layer has lready been cut but I still have clear trich on rest of buds.
I might feed just to see for even though airy buds my person meds level is fine.

What is your area, your dimensions? Also how far or closer are the LEDs and CFL’s. Exactly how many CFL’s? Do you know what your 5 watt LED system actually uses in real electrical watts? I assume they call it a 400 watt led because it uses 80 x 5watt LEDs to = 400, but the unit may actually be only using around 200 electrical watts or maybe even less. You may need a lot more light.

My led only pulls 175-200 watts and I can only use 5 26 watt CFL’ or temp gets too high and I have to run a/c.s.
The led is supposed to cover a 3.5 by 3.5,but I think only 3 by 3 is more accurate.My flower room is 4 by 4 but I get plenty of meds for personal use.
I do need more light and a better feed schedule.I’m sold on CFL’s for seedlings and sprouts,but I do need someting for two week and older veg stage.Just 3 by 3 is plenty for me.
Thanks for the help,