All the goals came in the first half at Underhill, with Jon Nurse replying for the Bees to make it 2-1 after half an hour. The result leaves Barnet bottom of the table with just one point from their opening three league matches.
"Both result and performance worry me and we looked nervous," said Robson. "There were a few strong words said at half-time and there is still loads of improvement needed.
"They had too much time on ball in the first half. We didn't get close enough to them. When we were in possession we didn't pass or move well enough. We were very static and never played the game we wanted to.
"We didn't get the ball out enough to the wings. It's something we have to address and put right - we are not creating enough chances.
"I don't want them just to lump the ball. If things aren't going right people get edgy. We have got to stick to the belief but we have to work harder. We have been in the lower regions for too long and we need to get up the league."