The England star has scored 59 times in three years at Old Trafford - but only once with his head.
"I'm not bad in training but then the games come and . . . I dunno.
"I've only scored one header for United and I haven't a clue why. But, hopefully, I'll get more this year."
Rooney insisted he has never been bothered about the number of times he scores.
He added: "There's more to my game than just scoring. I went 12 games last season without a goal but I set them up for others and we were still winning, so it didn't bother me."
Rooney is itching to get back in action after his summer break.
"They know all the right things to do in training and in their preparations but the main thing is their winning mentality.
"They've still got that desire and hunger to win."