Riyad Mahrez, Islam Slimani and Marc Albrighton struck for the Foxes.
Wagner felt a lack of focus and concentration in defensive set plays was to blame for his side going a goal down.
He told #HTTV: “I think in the first half we played well, we performed, we were very focused in the defence. We created good opportunities through Danny Williams, one he has to hit the target and the other he has to go for goal instead of looking for the foul.
“You have to be focused and concentrate to defend your set pieces. We lost Mahrez in this situation and we gave this goal away too easy.
“We know that in the Premier League you are not allowed to make mistakes and you have to use your chances if you like to hurt opponents. Today - performance wise - this was okay but these small margins we haven't done right in the offence and in the defence as well."