Each plant has it's own temperature, water and sunlight requirements so you really can not compare them to your other plants. It isn't just dark that they need, Moonflowers are tropical plants and need heat to germinate and to bloom, the air temperature needs to be consistently 70 degs or warmer day and night and the soil temperature needs to be in the 65 deg and above range. Not knowing where either of you live it's hard to determine what the problem might be but cool temperatures could be the problem. Most typically they do not start blooming on a regular basis until July, especially in northern areas. In Texas, they could bloom earlier. Also, if you are feeding them with a plant food high in nitrogen, that encourages vine and leaf growth but not blooms.