Two goals from Luton’s Jack Marriott helped the League Two side cause yet another upset in the first round of the competition.
After struggling to convert their chances in their Championship opener on Saturday, Bristol City fears they have problems in front of goal.
Cotterill said: “We put enough balls in the box and across the face of goal but we need to be putting those chances away. If we’d have done that, we’d have won the game comfortably tonight.
“If we weren’t creating the chances I’d be a little bit more worried. We’re still disappointed because we always play to win and nothing will change there.
“We’ve got near enough the same players and yet we’ve gone from a team who looked like scoring every time they took to the field last year to a team who look like they’re struggling to find their feet.”