Sculpting strokes are mirrored according to the local coordinates of the object. From manual about sculpting:
Mirror the brush strokes across the selected local axes. Note that if you want to alter the directions the axes point in, you must rotate the model in Edit Mode, not Object Mode
Hence if you rotated model in Object mode make sure to apply rotation (same effect as rotating in Edit mode, i.e. making changes to mesh data).
Otherwise the strokes might be mirrored not as expected (mirrored by X axis instead of Y axis or the other way around and so on). It's likely that strokes were mirrored on your mesh, but not where you were searching for them; e.g. if you rotated object 90 degrees by Z axis and enabled mirroring by Y axis, strokes will be mirrored along X axis instead.