Cube is active but not show up for Datasource list to create update rule

Hi, Expert:

We are using SAP BW 3.5. I built 2 cubes: cube 2 is the final cube, which loads data from cube 1 with some data transformation in update rule. Both cubes are active. I am trying to create update rule. But cube 1 does not show up in the pick list for selecting the infocube as the data source to create update rule. What should I do to make the cube 1 available as the source for update rule?

Thanks!

Jenny