The Denmark international joined the Blues last summer on a two-year contract but has since scored only three league goals in 22 appearances.
The former Rangers, Schalke and Newcastle man is now focused on working his way back into Lee Clark's plans as the club prepare for next season.
He told the Birmingham Mail: "It has been tough. It has been hard to just sit there and not get involved.
"I played the first game and then bang, I was on the bench and it's been stop-start for me ever since.
"I've experienced this situation at other clubs and you have to take it on the chin and keep working hard.
"I've just got to keep positive and hopefully it will come again for me."