Likely a group definition error. One thing for sure is that the first, upper left tile, can never be a random tile. If you have it defined that way, rotate the entire group by 90degrees, so, it moves that tile to the upper right hand corner instead, or rotate the group by 180 and paint that tile in lower right. All of the above assuming a 2x2 sized group. If the group is larger than that, the same upper left corner can NOT be a random tile and you will have to adjust the group definition so that the group is rotated.
Beyond that? No clue right off hand.
