Blocks mined with silk touch won’t stack

if I mine a block with my silk touch pickaxe and try to stack it with blocks that have been smelted or sheared they won’t stack. Here are the 2 instances I’ve had so far. 

1. Smelted cobblestone into stone. The smelted stone won’t stack with stone mined with my silk touch pickaxe. 
2. I mined some leaves with silk touch and they won’t stack with the ones that I have sheared.