embedded into each other, support gets weird. It could be that.
Bottom tile touching ground on one side and not the other, could be that.
anywhere along the way UP from ground, do you have a gap or misaligned block or window/door/etc? that can do it.
In general it all works the same, you get N tiles up from ground, and there is some sort of sharing side to side (so you can sometimes place one more block past what used to break if there are sides left and right that were not there before.
You can also add a little extra there, a little iron down below it may fix it.