The Show-Changes attribute uses the "item-hide" function. That function compares the current element with the previous element in the datasource.
Crosstabs are filled from the left to the right, so this function is useless for you. If you want that function, you have to write a new report function or rewrite the existing item-hide function to be aware of the crosstab structure. Then you would have to compute change information keyed by the values of your column dimensions:
On item advanced:
(1) Change the function to record previous values using map with a key that consists of all values of all fields used as column dimension.
(2) Compare the current value of your field with the previously seen value of the field. If there are no previous values, assume the field has changed. If the value has changed, show your element, else hide the element.
(3) Record that value as remembered value, using the column dimension as key.