[NTG-context] --|--- not transformed to endash|emdash

Hi all, usually, if one writes -- or ---, will receive real en-dash or em-dash, respectively. I just tried font called JohnBaskerville (commercial; from Štorm type foundry, CZ) but I got only '--' and '---', ie. sequences were not transformed to the proper characters. Dashes are present in then font, commands \endash and \emdash produce them correctly. What could be wrong? Best wishes, Tomáš