First, make sure that you have not frozen the drum track, becuase then no changes will take place.

Check to see if you have selected a "change" to the earlier drums selection.
Meaning you did not use the default choice - you chose a different variation.
There will be a line under a either the part marker or the chord at that spot (I don't remember which at the moment)...
I believe you need to "clear" that selection for any other drum choice to happen.
Do that by right-clicking the part marker (or maybe the chord?) where you want the change and select the "clear" option.