I am filling the bowl about halfway and am using a medium grind, stirring after each hit. I find that often I have small black spots on the bottom of my blend even after stirring after each hit and am having trouble getting the blend to cook evenly even with stirring each time.
Also, is it normal to get maybe only half a bowl on a set of batteries? Is it normal for the switch(with the spring around it) to be quite loose in its black casing?
In backwards order, yes the switch contacts need to 'float' inside the housing so they can make a firm contact, quite normal.
Try a
fine grind. This is a convection vape, only surface exposed to the hot air will vape. Only part of the load is exposed, at the bottom, near the holes, only a small percentage is, as you go higher, the air paths spread out and the flow is more uniform you're basically using the less random part, a
fuller load would also help in both volume and uniformity. The overheated bits come from being directly above a hole. If you let it go (don't mix) these lead to 'chimneys' of over done herb forming hit after hit. The shrink back of material only opens the channel up so the chimney gets wider and the condition worse.
With reasonable skills, good material and a little luck you should be able to easily do a full bowl on a set of batteries. OTOH, you can also run a pair into the mud without any vapor to show for it. 3 steps: Preheat, draw to working temperature, hit it like you mean it. Do 3 separate steps, don't cheat or rush it, you'll sort it out. Don't sweat battery life until you do. As Tim advised 'let it run' at step 1 until you're sure it's at full temperature (and maybe run it a few seconds longer yet to be sure), then and only then start the warm up pull without worrying about getting vapor (use 'test puffs' to sample until you do, the end of step 2), then dump what you have in your lungs and take a real, full, hit. Don't try to count the wimpy stuff you get working up to a positive test puff. After you sort it out, you can fine tune your technique to get faster hits using less battery.
Practice, practice, practice.
OF