Can No Longer Copy / Paste Actual Contents For Addition In Empty Cell

Quite spontaneously and for no apparent reason, I can no longer copy/paste a simple "add contents of 2 cells" to empty cells (e.g. e3+d4, to the next cells down the column, etc.). No matter what I do it will only copy the actual contents i.e. the result of the previous calculation. What I have tried:-

1. Deleting all contents and formats from the range of cells and starting over;
2. Using the drop-down menu of the paste button to select the "paste-special / formula" option (which keeps jumping back to the first default option at the top of the list);
3. Running the Office diagnostics built-in software off the microsoft top-left button to detect faults and anomalies - Nothing works and I am at a loss.......most peculiar...........