I just played through once, so I'm not an expert, but all the girls have 3 relationship quests, Relationship I, II, etc, and a love quest, named appropriately enough, Love. With Jean and Rogue, the relationship quests occur in order, then you get the love quest. With Laura, I got Love after Relationship II, and Relationship III happened last. All girls were ready for sex after the Love quest. For me, each quest triggered a day or two after the previous quest had completed. If it's not working for you, it does sound like the Relationship II trigger didn't work, and that would prevent the others.
Edit: Although I might be mistaken about some details. Laura may not have been ready until the end of Relationship III, which would put her on par with the others, i.e. after all the quests.