Two goals in five second-half minutes helped the U's come back from two goals down.
Cotterill said: "I've said from the first minute I walked through the door that we need to be more ruthless.
"We could easily have been 3-0 up at half-time, but we've failed to take enough of our chances.
"That is something we need to do better and, had we done that, we would have been out of sight and the game would have been over.
"I don't think we were at our best for the whole game and we got ourselves 2-0 up without really playing well.
"We were not sharp, but we've scored two goals and looked comfortable with 20 minutes to go.
"All of a sudden, we've got deeper and deeper and they have thrown caution to the wind.
"We've got an away point and extended our unbeaten run to five games, but we are all disappointed because of the way the game went.
"It's a great strike (by Sears) but we have allowed him too much time and space."
- Sports & Recreation
- Scott Wagstaff
- Gavin Massey
- Freddie Sears