Sum value isnt spilling over into next empty cell

[meljo1237]

When I type past the end of the cell my text is spilling into the next empty cell. When I add a sum to the cell, it wont spill into the next cell and just cuts off at the end of the first cell. Any reason why this may be happening and how to fix it?

Things to note:-
The next cell is definitely empty and the same format as the first cell.
The cells have no colour fill
I cant adjust the column width as that will interfere with other parts of the sheet.
I can see a blue outline showing where the sum will end
Cell format is general
Sum = data from another sheet.

 

[meljo1237]

Edit to add - its not spilling into the cell in the next column, not row.