In LibreOffice .odt file arrow characters replaced with box

When I open a LibreOffice (v on Windows 8) .odt file, arrow characters that were inserted with text, have been replaced with a box character. The arrows were chosen from menu, Insert > Special Character. All are Arial font. Sometimes the same file opens with the correct arrows showing, but more often it opens with the evil boxes. Is this a bug and can it be fixed, I wonder?