5
Are you doing this from admin? If so, there are 2 boxes to check, one is the Default theme box, where you set the theme and the other box is Selectable theme. If you don't set your theme as selectable there, then it still won't work as default theme in the earlier selection.
Cheers.
PS. Of course, if you are doing the theme selection with the block and you have already set all the theme as selectavle, then disregard the above and do check closely your cache and temp files.