A first-half strike from 19-year-old midfielder Moses Odubajo proved enough for the O's to progress.
"We created a number of chances and I'm disappointed that we didn't get enough bodies in the box to finish off the deliveries because could have won the game quite comfortably," said Robson.
"But we didn't and that's our story at the moment. We'll stay positive, we are getting into the positions to create chances and I'm sure the goals will come.
"They got their goal when their player got behind one of our defenders and we didn't defend the cross, which is our Achilles heel and has been throughout the season, although I think we are getting better at it.
"I'm desperate to give our fans something to cheer about, I thought we'd done it at Southend on Saturday where we played exceptionally well and I thought we did enough to have got some reward. It's difficult to keep lifting the players when things are not going your way but I know what a good group they are. We are just not putting the ball in the back of the net."
