As long as the bags are stacked in the correct order, you are harvesting the trichs (and any veggative material of that size) that were present on the starting material. What you get in your bags is what was there. Various strains develop different numbers of trichs of different sizes.
Generally, green hash means that some plant material is making it through with the hash. Trim from many mechanized trimming machines can be so chewed up that much of the green matter is pulverized so small it will contaminate the hash.
Shorten your mixing time, you can always do a second run with the material to extract anything left. Try a 6 or 3 minute first run. The yield will be lower, but the hash will be much more pure. Then do a longer run the second time if you wish.
Some strains are just not the best for hash. If you read through these forums and think you have the technique down pat, try some different strains.
Good Luck,
YBG