Input in Header not visible until clicked

Hello,

I've been dealing with this problem since yesterday:

I'm inserting a Checkbox in a header, and everything seems fine in the code, but when the page appears... the Checkbox doesn't show until I click the Header cell it's in.

Maybe some class or event is hiding the template until clicked? Or am I missing something?

Oh, and some other thing: I'm inserting the checkbox on (1,0) because if I try on (1,1) the checkbox is definitively not shown.

var chk = new AW.Templates.Checkbox;
chk.setId('chkHeader');
grid.setHeaderTemplate(chk,1,0);
grid.getHeaderTemplate(1).refresh();

Any idea is very much appreciated