While there are bugs in the present version of the Duel Engine available for release that prevent it from firing properly in situations where it should either never say anything at all or properly engage in a duel, the main reason it doesn't fire often is due to a bug in vanilla. Namely, liege levy subunits aren't assigned any commanders anymore. Therefore, there are much more rarely suitable targets for a duel in the few short combat pulses the mods get in order to hook into battle sequences.
AnaxXiphos has forked the Duel Engine and created HIP Duel Engine (HDE). Amongst the many improvements, full-on extensions and a new architecture, and collection of fixes that are WIP, he will solve this annoying problem to a reasonable degree, despite vanilla's bug. I will be lobbying Paradox to fix the actual bug at least before the next major DLC.