Thanks everyone for your comments on the last post. I spent a couple of hours with my clay books and my laptop and I think I have come up with an answer for the pieces that decided to pull themselves apart. As I said, they seem to be pulling apart at the seams, and it has been very random. All of the pieces were constructed the same way, I have ruled out the score and slip issue, I do that correctly, it was not moisture, everything is dried slowly and is bone dry when it goes in the kiln and I candle for a long time.
All of these made it :)
However, here is what I discovered:
these pulled apart... I loved these :(
I have been so puzzled by the randomness of this, but the article I found makes sense of this. The little houses that made it were all cut from the same slab in the same direction. The larger pieces that didn't make it got cut from different slabs and probably in different directions, against the grain.
This didn't happen to me before I got my slab roller because I threw all of my slabs and changed directions to get the thickness even. Since I got a slab roller I have been lazy, just rolling out the slabs and cutting out my parts. Somewhere in the back of my mind I am remembering a class I took years ago and the mention of rotating your slabs when rolling or throwing them came back to me. Ah ha! I am pretty sure this is the problem, so next round of slabs, I rotate them when rolling them out and we shall see if this solves the mystery.