Morning glories prefer to have their leaves in the sun and their roots sheltered, so it sounds like you're doing the right thing. Since this
plant is getting more sun than the others, it is wilting in order to reserve it's water supply (they lose most of their moisture through the leaves). It should perk up by the next morning, so you shouldn't necessarily have to water it more. As for not flowering as much, it may simply not be as healthy a
plant as the others. They prefer poorly nourished soil, and will
flower less in rich soil. Hope this helps!