Problem: When changing members in a slicer on a canvas, some gems does not change the corresponding values.
Make sure that the dimension exists in the where clause (or on rows/columns).
Make sure that the dimension hierarchy is not locked (on the gems menu in canvas design mode).
If using gems from different cubes on the same Canvas, make sure that the slicer is designed as a Shared dimension.
If typing your own MDX, make sure that it is possible to parse by the the drag and drop facility in the gems design mode.