The stone pieces, you have to click target the one you want it to meld to. If you place it any other way, it can leave a small gap, but if you do that, it will fill the gaps seamlessly. Ive had this problem off and on with both floors and walls/blocks -- it all does it if you don't pay attention to each piece as you place it. It will go seamless with all the buddies too if it is targeted right, so even putting it down in the middle of 4 others will work if you targeted one properly. The columns are the worst about it, from my experience.