I just found out why when I'm mining and try to pick Stone I can't, but with any other block in any other moment I can. I recently got Silk-Touch on my pickaxe and this stopped happening, I also noticed that for some reason I had a stack of, let's say, 46 Stone and another stack of 23 Stone, but I couldn't mix these. One of these stacks was Stone I got before Silk-Touch and smelted into Stone from Cobble; the other stack was a Silk-Touched stack of Stone. For some reason the game treats 'natural Stone' and 'smelted Cobblestone' as two different blocks/items.