The Robins, who had won their last five in all competitions, took an 18th minute lead after John Johnston skipped past two tackles, before finding former Buck Josh Hancock who slotted home.
Hancock sealed the win with a goal late on, after Connor Hampson's super curling free kick had got the hosts in front 16 minutes from time.
The Bucks chief believes his side had control of the game for large parts, but errors cost his them on a wet afternoon at the J Davidson stadium.
He said: "They were difficult conditions but they were the same for both sides so there is no argument there.
"I think we had a couple of chances in the first half we didn't put away, but I felt we controlled the game in parts.
"We were comfortable in open play and we nullified them for the most part, but we were the instigators of our own downfall, and that is not really us.
"We are quite a reliable team, individually and as a team.
"I was frustrated because we work hard through the week on specific things, and rely on the lads going over the white line.
"We did that in the first 18 minutes and controlled the game, and then four players have gone jumping and sliding into tackles and that is uncharacteristic."
Daniel Udoh drew his side level after 25 minutes of the game, but apart from that the Bucks never forced goalkeeper Tony Thompson into a save.
Cowan introduced Brendon Daniels and Yusifu Ceesau from the bench late on in a bid to grab an equaliser, but the Bucks got over run and Hancock sealed the win with his second.
The manager took the blame for the third goal, and said: "In the second half we took control again, and as staff we were talking about when we should go for the jugular.
"We tried to stay in the game as long as we could, and the third goal was my fault really because I have gone gung-ho.
"I felt we were in control of the game for parts, but we were not threatening in our attacking phases.
"In the final third it was a little bit hopeful, and we need to get back to the way we were and back to that belief."