This is by no means an exact answer, but my two cents.

Indica strains typically veg quicker and flower quicker than their Sativa cousins. I'm guessing that you probably just need to let your Sativa flower for 1-4 weeks longer than the Indicas. Hopefully someone can verify this for you.
Comatose Reviewed by Comatose on . Harvest/trichome issues 3 plants at 6 weeks flowing, 2 Afghani and 1 of unknow chronic origin. The 2 indica have lots of trichs, the one chronic, perhaps sativa hybrid doesn't have that many and harvest is a couple weeks out. Perhaps the sativa hybrid may need longer. Any comments? first 2 pics afghani, third and fourth sativa hybrid. Rating: 5