In some instances, stone made from cobblestone will not stack with other stone that was also made from cobblestone.  Stone from one stack can be crafted into stone bricks and stone from the other stack can be crafted into Polished Andesite. This appears to be a possible duplicate of MCPE-54971 and MCPE-54786 as well as numerous other issues reporting not being able to stack stone. 

In all other bugs it is reported that it's related to silk touch but that is not the case here. I have the same issue with no silk touch. 

https://bugs.mojang.com/browse/MCPE-54971
https://bugs.mojang.com/browse/MCPE-54786