For text to be centered, the width of the Label must be set. You can set it to percent value or points with percentWidth or width properties.
var _sln:Label = new Label(); _sln.text = _uofsModel.tempLessonTestResults.aUofS[i]; _sln.setStyle("textAlign", "center"); _sin.percentWidth = 100; _bgs.addElement(_sln); Using 100% width will center it in the parent container's x axis.