I dunno if this would keep it from getting suction, but I know I have gotten in a hurry several times now and left the low point drain cap off and of course when I turn on the water pump it sucks a gallon of antifreeze dry in a few seconds dumping it all over the ground outside
Just thinking out loud but maybe if the line is open it could be preventing you from getting suction? I am sure others will come a long with a better answer here soon.