I'm not sure about in Slick2D, but in Unity this sort of problem can be caused by the engine not assigning indices to wholly transparent blocks. For example, if tiles 2-4 are totally transparent, Tiled would assign the fifth block as index 5 but Unity would assign the 5th block as index 2.
Work-arounds may include slicing a dummy image with a solid color and then overwriting it with your final image to keep the slicing information metadata, or just not having wholly transparent blocks in your tilesheet.