[XeTeX] Absence of ligatures

[José Carlos Santos]

Hi all:

I have been working on a text whose main font is DTL Unico:

http://www.dutchtypelibrary.nl/Unico_rdrct.html

I have just noticed that the "fi" and "fl" ligatures are not created. To 
be more precise: if I compile this document:

\documentclass{article}
\pagestyle{empty}
\usepackage{xltxtra}
\begin{document}
\Huge
\fontspec{DTLUnicoST}
fi fl
\end{document}

I don't get the ligatures that I should get. Am I doing something wrong? 
Or could it be a flaw within the fonts that I am using?

Best regards,

Jose Carlos Santos