Yes, they need some sun to bloom. I don't know if the screen is blocking too much sun or not, but I would suspect so.

I have a friend that grows the most beautiful impatients, some are grown in a lot of shade. She claims the secret is to water them EVERY day and to fertilize every week with half the recommended strength, rather than the recommended strength every other week. Her impatients are always beautiful and lush and everyone who sees them are always WOWED!