It's IMO the sativa phenotype. I've got the same problem with my WWxBB and grapefruitxnev haze. For the last crop i had some sensi skunk #1. AFter 1 month on 12/12, i could see only the bud sites but no real buds. After 2 month of 12/12 the latest had been finally the fattest, believe me. I don't believe me myself, because i'm worry about my crop like you. Can you see the trichs ?
Fhydro