Im no expert on the use of the HG/DB method of roasting, use a slightly modified popper to keep me going.
It sounds from your description though, as though you are taking a bit too long to get the beans to 1st Crack. If it was me, I would be trying to get to first crack in under 10 minutes, probably at around 7-8 minutes and to 2nd Crack in roughly 5-6 minutes after that.
It could be that when you move the HG closer and ramp the heat up at the same time, you are accelerating the roast process just a bit too quickly at the time when you should be exercising some level of restraint in order to maintain control of the roast. Why your first batchs aroma should be significantly different to the seconds could be due to any number of things occuring, including under/over roasting.
The only time Ive experienced what you describe is when I over-roasted one particular batch of Brazilian Santos beans but in my case, there was plenty of oil evident on most of the beans even though the bean colour wasnt all that dark. Maybe one of our more experienced contributors could offer more focused advice for you. All the best,