Based on specifications for Orbits plastic auto drains, they open/close at pressures in the area of 3-5 psi. A 10' tall water column has around 4psi, and you say you are running a multi-level system. So it is quite possible that you are operating right at the cut-over point for the auto-drain.
Now I personally don't like auto drains. I instead use check valves (that can hold up to a 25' water column) to keep my pipes full (so all that water isn't wasted each cycle) and manually drain the system at the end of the year.
But if you really want to use auto-drains, it sounds like you will need to install multiple ones. Install a second one about 6' higher than the current one. The higher one will drain the water higher in the pipes. Once the higher water has drained, that will reduce the pressure on the lower drain and perhaps allow it to drain.