Hey Bassman -
You might want to check closer to where your grade a buds are (maybe you did). I usually snip a sugar leaf from a nicer cola, and one mid plant, one low. Sometimes the smaller lower stuff doesn't amber up as quickly in my experience. I wouldn't be suprised if the trichs up top on the prime buds have more amber - but maybe you've checked that.
Also, there are definitely strains/phenos that don't amber as much or even at all. But, they are done at some point.
Sometimes, I go more by how done it looks in all other respects, in conjunction with the amber trichs (for what it's worth, to me, they look like another week or two would do them good but it's hard to tell in the pics sometimes).
A test smoke/vape can help decide, too. Taste will be meh, but effect is usually present. Just a few thoughts...
You can also give them a nudge by going to a longer dark period, sending them a stronger signal that the end is near, time to ripen up (if you want)!
Oh, and 50% amber is a pretty heavy stone -- to me at least. Personally, I prefer around 10-25% amber, mostly cloudy.