Manager Gary Johnson admitted a poor second half had cost his side as Exeter City claimed the points in a seven-goal clash at the LCI Rail Stadium.
The Robins had produced an impressive opening 25 minutes in the Sky Bet League Two encounter with clinical a brace from Kevin Dawson putting them in control.
But goals from visiting captain Jordan Moore-Taylor and Reuben Reid saw Exeter strike back and draw level before the break.
Experienced front man Reid grabbed his second of the match in the final quarter from the penalty spot – with Robins reduced to ten men after Jamie Grimes was sent off – before Jake Taylor raced away for a fourth.
The home side did hit back with Dan Holman setting up a grandstand finish with his first goal of the season in injury time. However, the gaffer refused to pull any punches after the game.
“We showed what we can do in the first 25 minutes and started brightly, but at this moment we can’t do it for 90 minutes,” he said.
“When you go 2-0 up you know teams can come back to 2-2. But I wanted to see the character and passion and commitment to not let the game get away from us and I felt we were poor second half.
“Sometimes you make individual mistakes – we are not happy with the penalty and sending off. But as a team we weren’t potent or showed enough intent in the second half.
“We need more than 25 minutes. The lads have had a few home truths and we are going to be working hard these next few weeks.”
Johnson also revealed defender Jordon Forster was set for a spell on the sidelines after being stretchered off in the second half.
“We will confirm what has happened when he has had scans. But the physios believe it is an Achilles problem that will take a little while to recover from,” he added.