Align UISlider thumb image - objective-c

I've customized my UISlider but my thumb image looks strange, I mean it's position not aligned by center:

But should be like this:

And here is code:

UIImage *leftTrack = [[UIImage imageNamed:@"blueTrack.png"] stretchableImageWithLeftCapWidth:3 topCapHeight:0];

UIImage *rightTrack = [[UIImage imageNamed:@"whiteTrack.png"] stretchableImageWithLeftCapWidth:3 topCapHeight:0];

[slider setThumbImage:[UIImage imageNamed:@"thumbButton.png"] forState:UIControlStateNormal];

[slider setMinimumTrackImage:leftTrack forState:UIControlStateNormal];

[slider setMaximumTrackImage:rightTrack forState:UIControlStateNormal];

Where could be the problem?

EDIT: I found solution of this problem. Thumb image background was transparent, and I've not seen that thumb image was not aligned by center: