I'd suggest it's not stacking as such, it's likely the inefficiency of such a short draw (e.g Your body isn't working efficiently at that draw, not using shoulders or back, can't get into the bow).
The only way you'll actually see if it is stacking is to plot a force draw curve.
The string angle (angle between sting and bow tip) is no where near 90 degrees which is the point after which one would expect stacking would start.
The outer 1/3 of each limb could flex a little more. A pyramid design will tend to be stiff in the outers due to the physical limitation of having to have the tip big enough to make a nock, where it should theoretically taper to zero width for circular tiller.
This video shows how it works...(you may want to fast forward through to about 4:55!)
https://www.youtube.com/watch?v=WdQCB5Txv_EThe way to get it to draw more is also to simply pull it further! If it's Pacific Yew, it should be able to take it.
Del
PS. I once had a guy trying to draw a warbow say to me. "Wow I can really feel it stacking"
I replied , "That's not the bow stacking, it's you!"
He'd just reached his physical limit.