Could be a million reasons, lighting, improper diet, bad temps, bad root system, disease, etc. its also possible you just harvested early. ganja really does most of its "filling out" during the last couple weeks. Its not uncommon to get excited and harvest too soon.
The grassy smell is chlorophyll and if you cure your bud it should dissipate after a couple loong weeks.
My recommendation, give the FAQ a real thorough read, know your bud inside and out, learn as much as you can, be patient, watch grow videos, talk to anyone you can who has grown, take all that information, make it work for you and your next grow will turn out amazing.
Good luck and happy growing