What determines the amount of oleo saccharum that you get when combining the lemon zest and the sugar?

Lately I started making the “State fair lemonade” from food wishes and learned about oleo saccharum. The first two times I made the oleo saccharum I peeled 6 lemons and used 250 grams of sugar (which was about 1 1/4 cups). I let them sit for about 12 hours or so (I think even more) and I got quite a bit of yellow liquid. These two times I didn't press the lemon zest, just left it under the sugar.

Now the other two times I did the same, 6 lemons and 250 grams of sugar but in these cases the result was only the sugar getting kinda wet and that was it. Ultimately, for making the lemonade it didn't make much of a difference as I ended up boiling some water and adding the mixture. But if I want to make this for other uses, I want to understand what causes you to get more or less oleo saccharum. Is it in the lemon type? Is it in waiting a couple of days before using the lemons? Does the shape of the container (meaning sugar covering the peels completely vs touching most of the colored side) have any effect of this?

I tried searching answers to this but everything that I was finding didn't mention issues like the one I had.