Rotate function stops working

Hello,
I’m making tile sets and as I begin a new sprite I can easily select a tile set and rotate it by copy+paste then selecting the tile and using the drag arrows. However, as my sets get more detailed, the ability to rotate a tile stops working. I can no longer rotate it via any method, dragging or using the tool bar edit->rotate. I assume I’ve changed some parameter that of which I am unaware. Any ideas?