So I am personally not an Aquavitro 8.4 user but I did a little digging into the instructions and FAQ and found this:
"How much will one dose of eight.four™ raise my alkalinity?
A: Each 5 mL of eight.four™ added to 80 L (20 US gallons) (standard dose) will raise alkalinity by 0.25meq/L (or 0.7 dKH)."
So 5ml:20gal will increase dKH by 0.7
So if you dosed 10ml into your 40gal you should have increased your dKH by 0.7 as you increase both the ml and gal proportionally.
What I would do to verify you're seeing an increase after dosing is:
test the aquarium, dose the 10ml (assuming you're aiming for a .7 increase), wait about 30 minutes and test again. The results should be ~.7dKh higher.
On another note, what dKH are you targeting? I suggest trying to maintain whatever your salt of choice mixes to.
For example, I use Red Sea Coral Pro which at 34ppt is supposed to have 11.5 dKH but regular Red Sea Salt (aka "blue bucket" ) at 35ppt has a dKH of 8.4 I think.
Edit:
You didn't note initially when you were testing, so if you tested once before you started the doses then once after it is possible that coralline algae or something else another process is consuming it. You don't mention any skeletal corals but maybe snails or something else along those lines.
