Assuming that you have a full reservoir if you open the bleeder 1/2 a turn or so and nothing comes out then try actuating the rear brake lever one time.
If nothing comes out then it may be as simple as a plugged bleeder.
To check, pull the bleeder all the way out and see if the small cross drilled hole is plugged up.
You should be able to blow on the end and air should come out the bleeder.
Let us know what you find.