The O's, who lost 1-0 at the Matchroom Stadium, are now four points adrift of second place in League One.
"I thought we were the better side, but we didn't do our jobs from a set play," Slade told reporters following the game.
"The fans were excellent all game. They urged us on and we just couldn't find a way through."
Orient return to action next Saturday with a trip to face Sheffield United.