Why are vertical table borders not visible in presentation mode?

I have created a table with dashed cell borders. The borders are visible in normal (editing) mode, but when I go to presentation (full screen) mode only the horizontal borders are visible. How do I fix this?

LibreOffice Impress on Linux Mint 17.

It looks like I may have had the line width set to low. At the default setting of 0.05pt(!!) it displayed the horizontal lines but not the vertical. Once I increased the width to 0.5pt everything is fine.