Sainz finished fifth and Leclerc seventh in Miami as Max Verstappen led Sergio Perez to Red Bull's fifth win in five races and fourth one-two.
"(It's a) very similar picture to the beginning of the year, we are competitive in qualifying (but) once we come to race day we are struggling like crazy," said Leclerc, who had been on pole position at the previous race in Azerbaijan.
He had finished second in the sprint in Baku, raising hopes that Ferrari had made a step forward, and a lonely third in the Sunday race.
Leclerc crashed in both practice and qualifying, when he looked competitive, at the same corner in Miami and said he had struggled with the car bottoming at speed and also affected by the wind.
Leclerc was Verstappen's closest rival last season, winning two of the first three races before the challenge petered out in strategy errors, unreliability and driver mistakes.