The Baggies had sailed into a 2-0 half time lead against the Royals in their FA Cup 3rd Round replay at The Madejski Stadium only to be knocked out of the competition by a Leroy Lita hat-trick for the home side.
"It is disappointing to go out having played pretty well in the first half," Robson said after the game.
"At 2-0 up at half-time I asked my players to give me more of the same, but to try and keep the ball more and they just couldn't do that.
"Credit to Reading, they stepped up a gear and scored an excellent second goal, but the players we had out there should have been capable of better than that.
"The only consolation is that we now have only Premiership games to concentrate on and prepare for between now and the end of the season.
"A Cup run can bring injuries and suspensions but it doesn't matter what competition you are playing in, an unbeaten run brings confidence, so this is a disappointing defeat."