If you are setting the contents of the cell via a formula or VBA code
make sure that you have changed the formatting for the cell also. Check
that the <right mouse click Format Cells / Alignment Tab / Wrap Text
check box is selected.

For example, I created a formula for a cell:

="1st line"&CHAR(10)&"2nd line"

This appeared as "1st line2nd line" on a single line in the cell
initially. Once I changed the cell formatting (as above), the cell
appeared as two lines with the 'split' as required.