Well, if you take the rest of the community's word for it, you'd get told something like "itz cuz ur gey" or "you need to 'git gud' before complaining like a salty casul." Realistically, however, it is highly possible that was a bug in her triggering, which is known to be a bit glitchy. You could also check your inventory to see if you scooped up the Broken Pendant, as that activates the DLC, during which time Dusk is definitely not summonable. In any case, it is not a game-breaking issue, and until you rescue Dusk in the DLC, Elizabeth can effectively act as her substitute as a spell vendor (they sell the same spells); you find Elizabeth immediately after Sanctuary Guardian in the DLC. I know I'm not 100% satisfying your question, but hopefully this is a helpful pointer.