Harvesting help needed please

This is the first time I have grown weed and I'm in need of some advice. They've been on 12/12 for 6 weeks. I was planning on harvesting in 2 weeks. I have a couple of concerns if anyone could help?

1. The middle buds seem to be done but the top of the plant is still growing new bud. Is it okay to harvest the middle section now and the top later?

2. Why is the the plant so sparce at the top? I've checked the ph level and it's 6.5 ish...

3. Any ideas on yield? I was hoping for 2 ounces per plant. Is that unrealistic?

The first photo is the top of the plant and the second photo is the middle of the same plant...

I've got 13 in total and any comments would be appreciated. There are only 3 plants that are lacking bud at the top. The other 10 are really good. That's the main reason I am concerned. The three I mention are also a good foot taller.

Do taller plants take longer to flower?

Thanks in advance...

not sure on your other Qs but 2 ounces is easily achieved i'm told, it's what im hoping for, good luck with your grow

None of em look done to me, I would say 3 to 4 more weeks

But good job thus far!

They looks great, but not anywhere near done. Give these sativa leaning plants another 6 weeks, and when you savor a joint you'll know it was worth the extra time ripening.

6 more weeks? Bad times...

Generally speaking, taller plants tend to be Sativa dominant, and they do take longer to flower. The top cola on a healthy plant, will continue to put out new, white pistils, until the day you chop her, because her main goal is to get pollinated - and dry, dead pistils won't achieve that. Looking at your pics, none of your bud is done - not the middle buds, either. I agree with the others - you still have several weeks to go. They look lovely, by the way!

thanks for the input

those look pretty damn good! get yourself a dope scope and start checking your trichomes, they'll tell you when to harvest. a lot of people harvest different parts of the plant as it ripens.