I cannot use align menu when centering a text box, suddenly

I was working my text boxes with text in it. I could center the text boxes easily from the drop down menu. The item menu is align and gives you the possibility to choose between left, center, right for examples. Now I cannot anymore, since it is greyed out. The text box is of course selected ! What is the problem ? Few minutes ago, i could do that.
Thanks for you help !

Nicolas