The Complete Composites Tutorial thread will tell you more, and the PDF file linked to at the end of the thread goes into the technical details of how it works.
If some of your composites are blank or incomplete, probably the diacritics used to compose them are missing or empty.
Some use a combining overline, which will be absent for most users, but there should be a fallback glyph (the underscore character). If that’s also missing, then add it.
For exampe: g stroke is composed of lower case g + combing long stroke overlay (decimal code-point 822). If the font doesn’t contain that glyph, the underscore glyph is used instead (decimal code-point 95)
<Composite><!-- g stroke -->
<GlyphMapping>485</GlyphMapping>
<Member id="1">
<GlyphMapping>103</GlyphMapping>
<UseMetrics>TRUE</UseMetrics>
</Member>
<Member id="2">
<GlyphMapping>822</GlyphMapping>
<GlyphMapping>95</GlyphMapping>
</Member>