That's because a harvester in shearing mode will never get any saplings.
My setup is one planter with two harvesters, the one shears leaves, the other doesn't. When a tree grows it's basically random which one gets to cut it. The setup works well for me.
Or you can always setup two separate tree farms.
Thanks for the quick reply!
I was actually thinking of 2 separate tree farms. 1 farm would basically just be there to collect as many saplings as possible and then pump the extra saplings into the other farm designed for sheering.