What dialog option are you using? dbu or pixel? The default is pixel if you don't have one specified. The point I'm trying to make is that if you're using pixel, you're going to have to make the tabs larger in size for them to be visible.
Aslo, it may not have anything to do with it, but I see that you have the ID '15' for the first tab, try creating the tabs before all over controls.
tab "", 1, 1 1 100 100
tab "", 2
