In many cases, the top tank will only be about 1/2 full when everything is stabilized. Check it cold! Some might be a tad lower, but being too full when cold will lead to what you describe. If you add an overflow jug, mimic the location of the OEM units, hose position included, so fewer air pockets exist in the hose itself.
On cross-flow radiators with their side tanks, it's not uncommon to see the first three or so cores uncovered when the coolant level is "where it runs". Unless you have a coolant recovery system in place.