Boss Gary Johnson bemoaned his side's defending as they slipped to a second successive defeat in four days, this time losing to Grimsby Town at the LCI Rail Stadium.
The visitors carried a two goal advantage into the break despite Harry Pell's 26th minute equaliser, his first of the campaign.
The midfielder's third goal in successive home matches against the Lincolnshire side got the Robins back on terms, but two goals in as many minutes with half-time approaching put paid to any comeback on the night.
A late Carl Winchester goal gave the Robins late hope of salvaging a point but Russell Slade's side held on to secure the victory.
"It's no use throwing the ball in there with 10 minutes to go, trying to get a couple of goals back," the manager said.
"We started off too slow and too weak and they were winning the battles all over the field, and that's why they got their opportunities.
"We defended like schoolboys as a group, so days off are cancelled for a little while and we've got to get back to the good defending we had three days ago.
"It's the level we're at and Grimsby will have a day like this one day, and we'll have a better day.
"We're disappointed we've let our run slip but we're a good group of coaches and we'll work as many hours as we need to to get their confidence back."