I have got an OpenType Arabic font with a good number of ligatures. But some characters that share the same body with others with a different number/placement of diacritics (dots etc.) do not have the ligatures that those others have. I want to create those ligatures by duplicating an existing ligature glyph and changing or adding diacritics with Fontlab Studio 5. I tried to do this intuitively, added those characters in substitution features, but the resulting font became unusable with letters not joining. What am I supposed to do?