If you have a light meter or camera, take a light reading during the brightest point in the day under the burlap. That should give you a better since of how much light your orchids are getting. Cattleyas need between 2000 - 3300 footcandles of light to bloom. They can grow in much less light, but are not as likely to bloom. Vandas will need even more light.
Cheers,
BD