The defending champions fell further behind Manchester United in the Premier League after being held to a draw by Sam Allardyce's team at Upton Park on Saturday evening
Both sides were held goalless after Kevin Nolan saw his strike incorrectly ruled out for offside in the first half.
The visitors improved their performance after the break, but ultimately failed to hit the back of the net despite their array of attacking options.
"I don't think it was a fair result," Mancini told reporters. "When you have 65 per cent possession, 22 shots on goal, and we're missing four incredible chances to score [you deserve to win].
"I'm happy for the performance because we played well and I'm unhappy for the chances we're missing. When you're missing incredible chances it's frustrating but the players are like me, but this is football.
"I think that when you win the title, after [the second season] is more difficult. We've had some problems, with important players injured, in the first two months and this maybe is the biggest problem we've had.
"We conceded a lot of goals at the start of the season but we've improved our defence and we need to start scoring like last year. We need to work on this."
West Ham boss Sam Allardyce was content with the draw, but feels his side should have won after Nolan’s early goal was ruled out.
"What I want is for a goal to stand when it is a perfectly good goal," he told ESPN. "At such an early stage it could have given us the momentum to carry on. In the end we didn't need it because we had a fantastic point today."
Nolan was equally frustrated with the decision, adding: "We were very unlucky with the goal, it wasn't offside as we know but it's one of those things. Unfortunately the referee has had it wrong but hopefully we'll get one [in our favour] later on in the season."