As you probably know, ligatures need to be defined in the font as glyphs and in layout tables. AFAIK, ADE supports the fi and fl ligatures if the font supports it. Some fonts also support the use of
ZWNJ and
ZWJ between two letters to enable/disable optional ligatures. (You could try inserting a ZWJ between f and i to test if that makes any difference.)
Also, to exclude font issues as the cause of your problems, you may want to repeat your tests with a commercial font.