Think they need permission for some the u18 games, especially away. Teams like Arsenal don't tend to have their games shown live, think City as well. Also don't show live games that clash with the first team. So when the u18s used to play a lot of saturday afternoon games they missed a few, they've scheduled a lot more Fridays to avoid this by the looks of it.
For the u23s they've only not shown 3 games. 2 as they aired the u18s instead and one as the first team was playing.