If you close the valve on the vent pipe up top and secondly plug the outlet, you can then return to the vent pipe valve, open and watch for air to escape. This would at least verify that for some reason you are not getting all the air out of your system to start the siphon.
If air escapes when you open the valve, remove the outlet plug and see if it's siphoning. You'll know when it is.
Be safe.
