Never made Manchego , but have made a few Cheddars , in my opinion , Cheddars will be bland at anything lees than 6 months , no matter what they tell you in the book.
Perhaps you're just not letting them age long enough , and I stick with traditional Cheddar now , they always seem to have way more flavor than Farmhouse Cheddar or any other shortcuts.
That's been my experience anyway.