Those pics don't show anything useful as far as this issue is concerned..
(But that power strip scares me being so close to the sump/moisture..corrosion over time may be a serious issue/fire hazard..)
So you have a "bean" setup?
Outputs of drains 1/2" or so below water level in sump?
You should have no problems with a mag 1200 and 1" piping.. But I can't comment on what the issue might be without additional pictures/information..
Do you have a valve on the "full siphon" channel? Crank it wide open and start from there.. It should have zero problems wide open and will need to be closed some I'd suspect..