3 Weeks into flowering?! I'd say it's because you have poor lighting, Ive helped a friend grow that exact Nirvana strain outdoors and we got 5 ounces per plant....

Im not dissrespecting your budget at all...

Try outdoor, skunk rocks outdoor, but it smells..

Your plant is looking awesome though, i mean it looks very healthy, but the buds just arent comming too quick, give it more time though, sometimes buds produce the most afew days before harvest!..

well goodluck with that.