Selecting all cells. Cells.Select is not working

This line only works when the 1st sheet is the active sheet. It doesn't work when the others are active.

Cells.Select


When the 3rd worksheet is selected, this isn't working on the 1st sheet:

Worksheets(1).Cells.Select


And to further complicate matters, these things do not ALWAYS work or fail. I'm not sure how to explain this. Any ideas?