OK, I looked at mine and it does say thermal air flow on the box. Best I can figure is this describes the warm air that escapes the top holes (the 4 small ones, not the ones with plugs) which then draws the fresh air in through the bottom holes.
That sounds right on the wafer, but it's been a long time since I put mine together. I'm a major over thinker and seeing the separate pieces means it took me forever (months) to take the initiative and put it together. When I finally did, I couldn't believe how simple it actually was. Point being, if you've got your temp stable, try not to worry about it. Sometimes when you tighten the wing nut it will slightly change your little lever. So I get it good and stable before tightening and then make sure it doesn't budge while tightening.
I don't have tons of experience hatching, so please feel free to seek a second opinion. In still air you should be running at 102. I had mine spike to 104 and go as low as 98. I would not let it below 98 if I could help it, or above that 104. I know that depending on the stage of development the embryo can withstand higher (I think early stage) or lower temps (I think later stage). I also know that higher temps may cause early hatch and lower can cause late hatches. When hatch day comes, it can tell you which way you were on the scale and maybe adjust for your next hatch if needed.
Even though you say you get a different temp within a matter of minutes, I wouldn't count that until at least 1 hour after the adjustment So you can see what the range is for real verses just when the wafer turn off or on. Also, where in the incubator are you measuring your temps and are you using the thermometer that came with the bator? Are you using a secondary thermometer or hygrometer? The one that came with mine registers about 2 degrees warmer than reality (if I remember correctly). And I put a second one in and they never agree exactly. But they are at different places. So it is important to move the eggs to a different portion of the bator on occasion. I do this with one of my daily turns moving those from the outer edge towards the middle and vice versa as well as to opposite sides including top and bottom. This helps to ensure that 1 egg isn't constantly in a warm spot and others in a constant coolness, which could cause a staggered hatch.
One other thing that could effect your hatch is the eggs. Where are they coming from? Fertility may not be high. Also, you can improve hatch-ability by giving higher protein feed than layer, if the eggs are coming from your flock and that's what you use. 20-22% is a good protein level for breeders according to studies (and good breeders). It gives the hatching chick more nutrients and a better start. It's the amino acids in the protein that makes the difference.
Were you able to diagnose your previous failed hatch? Did you have development/fertility? Did you get any pips at all?
If you haven't checked out the hatching 101 article in the learning center, it is loaded with tons of great information!
Hope this is helpful!
