Are you using any veg nutrients or only bloom nutrients? If your nutrients is bloom only that could be your issue, my buddy had the same issue all the way through harvest and his buds were airy
What is the name brand of nutrients? What type of water are you using? Are you PHing your water after adding nutrients?
I believe it is also time to transplant. When you do, use a potting soil such as Fox Farm’s Ocean Forest or Happy Frog and go up to a 3 gallon pot. When you water only use 4 cups per pot. This way your plants won’t be relying only on your ability to use the right amount of nutrients, because coco has none. In fact, with those potting mixes you wouldn’t really need to add anything although you still may if you desire. Its foolproof unless you overfertilize and burn them.
I like Success Nutrients; Micro, Trees, and Flowers. These three must be used together and its easy.
It is best if you use purified water which has no nutrients (15 ppm). You still need to PH the water. If you add nutrients you still have to PH the water. Here is a chart from Oaksterdam University. Be sure the ph is 6.5 or 6.6 for optimal nutrient uptake. I believe your problem is nutrient related. ph_chart|375x500