A last-minute leveller salvaged about for the Bucks after two Boston United goals cancelled out Daniel Udoh's early strike.
And Cowan believes the opening goal should have given his side the impetus to win the game.
"I’ve got very mixed feelings about today," he told the club's video service.
"I thought we were very much in control in the first-half but we needed to be more ruthless and clinical.
"We weren’t very efficient, but I think because we were so in control, a bit of complacency sets in.
"They need to understand that we need to put games to bed. When we’re controlling games and being pretty dominant, we need to put games to bed and we should have done that by half-time.
"You’ve got to be ruthless. You’ve got to make sure the other team don’t get any momentum and they don’t smell blood."
Despite the result, Cowan admitted he was pleased with his side after pegging the hosts back.
"It’s clear to see that these lads always make sure they give everything and that was apparent.
"A draw today was the least we deserve but we need to be better."