I have occasionally seen a condition like this. it could just be that the holes in the calipers that the bleeder screws screw in to are plugged. Without any pressure applied, remove the bleeder screws completely and insert a small pick into the holes (anything of appropriate size and strength should work), then reinstall the bleeder screws and try again.
ps also it could be the bleeder screws themselves are plugged. At the bottom of the bleeder screw (below the threads) you will see a hole drilled through which the fluid passes through when the screw is opened, are these cleared out?