I've had multiple saves with 2+ years on them where no mushroom trees have spawned, even when I've gotten lazy about clearing trees. It's totally normal to not have it happen!I don’t think this is the issue. For over 6 years of game of time you couldn’t move through parts of the farm because of the number of trees growing on about half of my farm. With so many years of game time, so many trees and a 5% chance every day in the Fall for a tree to turn into a giant mushroom this is almost statistically i possible. My other farms had giant mushrooms between years 2-4.
Worth noting Miretar is playing on iPad so this option isn't available just yet, but it will be in the future. And it's Qi who sells the Mushroom Tree Seeds.If you're that desperate I believe the new shop on Ginger Island sells mushroom tree seeds, just beware of the parrots.
I found it easiest to do in a 4-Corners farm. You just leave one quadrant blocked off. (There are logs, stumps and boulders blocking access except through the center of the farm.) In that quadrant, just let trees grow. Just let it happen, until the place is choked, like you can not get through it at all. Let fall happen. I've played a couple 4-Corners farms (my fave map, hands down) and this has never failed me there.I have played many 6 year + files since launch and still havent seen one until buying an actual mushroom tree seed