Premise - You cannot cover all the angles
None of us wants to leave an open court; a spot that the other team can attack and beat us. Got passed down the line? &r#@o! Coudn’t catch up to the cross court angle? $#*&@
The reality is that there is always going to be an open angle on the court. Your job? Make the open angle as small (and thus, as difficult to hit) as possible. Do not stress if the other team finds an opening - as long as the opening is the smallest one you could leave open.