Better enemies will more likely block and riposte than normals…
Also, your warfare skill matches up to your enemies warfare skill and if theirs is higher than yours they will have faster strikes and block better than you.
You need to learn to use feints to throw enemies off, attack in opposite zones of where the enemy guard is, and learn patience.
Practice combos with Bernard, combos are invaluable vs enemies that block often because they allow you to penetrate their defenses.