The City Ground team were very weak when it came to letting in goals from dead-ball situations.
Nuno Espirito Santo’s side conceded 22 goals from such situations last term.
"We tried to change the structure - man against man, zonal - in the end it is not about the structure it is about commitment," Sels told the BBC.
"I'm not saying that we didn't do it well last season but in the end it was crazy, we always took a goal from the set-piece.
"In football, set-plays and set-pieces are important. We need to improve this for next season. Last season every week we were working on it but we still conceded goals on it. There was not really something that we could do better, maybe just commitment, so maybe this season we won't concede that many more goals like that."