Stuart Lancaster's side ran in seven tries in Rome, Leonardo Sarto the only try scorer for Italy, which means Brunel's outfit lost five from five games.
That marks a disappointing return for the Azzurri, who last year claimed the scalps of both France and Ireland on their way to a fourth-placed finish.
The Italians did push Wales hard in their opener and also narrowly lost to Scotland but Brunel can't put his finger on what went wrong.
"If I had the answer would be easy. I cannot answer it right now," said the Italy coach.
"We must see if we can maintain our ambition to play, some players do not yet know their future, and confidence in the future is essential for an athlete."