As you can see, one of the cells has no border. That is because it is empty. Try to insert a space in the cell. Still it has no border.

The trick is to insert a no-breaking space in the cell.

No-breaking space is a character entity. If you don't know what a character entity is, read the chapter about it.

The no-breaking space entity starts with an ampersand ("&"), then the letters "nbsp", and ends with a semicolon (";")