Same issue for me; I do not have the option to upgrade the youth facilities further. Although the "maximum" level in my save is "average". I have uploaded the save game named "Șerban Ditu - Unirea Slobozia.fm".
My other guess apart from the incremental issue which has been mentioned is that maybe the board vision lacks something along the line of youth development. I haven't been able to negotiate that with my board since they insist on buying first team ready players.
Anyway, hope this helps. Thanks.