I have read Section 13 of the rules and the way I understand it is as follows:
If a German unit or stack occupies an allied entry hex, then no allied unit can enter the map in any entry hex in that sector until all blocking positions for that sector have at least six allied factors and the units in at least one of those blocking positions has to dislodge the occupying German units by attacking them and forcing retreat or elimination.
Is this correct ? If so, pretty harsh on the Allies !
I think the important part is in 13.8 Allied Attacks onto the Map:
RESTRICTION: An attack onto the map is only allowed if all German-
occupied Entry Hexes in that sector are blocked (have at least 6
Allied Defense Factors assigned to them). In other words, you can’t
attack onto the map until your flanks are covered.
This seems to me as though you only need 6 allied factors in as many blocking positions as entry hexes are occupied by German units in that sector. So if the Germans have occupied two entry hexes then the Allies need 6 factors in the two blocking positions corresponding to those entry hexes.