Bluebirds may not understand how to use the bluebird feeder you provide them. Place the dish on top of the feeder to train bluebirds. Once they notice the feeder, they will go into the feeder. Besides, if you set a bluebird feeder with plexiglass sides, bluebirds are probably trying to go through the glass rather than the feeder holes. Once the bluebirds can go inside, they will understand how to enter the feeder.