The young Spanish striker spent the entire 2010-11 season on loan with Bolton, with his parent club Benfica keen for him to gain some regular football.
Wanderers were keen to make the move permanent, but have been told by the Portuguese club that the 20-year-old is not for sale.
"I have tried everything to push through a deal for Rodrigo but the president of Benfica says he is not for sale at any price," Gartside told O Jogo. "President [Luis Filipe] Vieira says it not a question of money and that his decision is based on sporting factors.
"Benfica do not want to part with this player because they have high hopes for him.
"If Rodrigo had been with us this season, I am convinced that our situation would be different. Bolton need his goals but it is not possible."
Rodrigo scored one goal during his stint with Bolton.