The on-loan West Ham striker thought he had opened his Eagles account against the Robins only for referee Rob Shoebridge to rule it out despite the ball clearly crossing the line.
Sears is still searching for his first Eagles goal two months later and Warnock said: "Since the Bristol City debacle he's not firing and it's knocked his confidence a bit.
"He was really down after that carry on and hasn't scored a goal since. It's not rocket science really. He's got to find the back of the net somehow.
"At the moment he's waiting for things to happen and when you're playing well you make things happen yourself. He's not producing what we expect of him at the moment but he'll come back, he's only a young lad."