Are you able to just pick up a grown tree and plonk it down in the same square though? I have a fruit orchard area which is just a butt load of fruit trees thrown on a hill, but I didn’t grow them, I just placed them up there. I’m not home to check the grid on my map but I’m so sure there’s gotta be more than 12 in one squareMy guess is too many in the same square on your map. I had the same problem when I tried to grow 16 in a square...I believe 4 didn't grow at all.
I planted all these fruit at the same time. All but the two in the middle are growing. Any idea why?