New topic no styles

I have a weird problem.  It worked before, now when I create a new topic, none of the styles from my stylesheet are applied, just the default ones.  This used to happen automatically.  As a workaround I can manually set the stylesheet property in Properties every time I create a new topic, but I would like to find out how I can fix this.  Is there a file somewhere which controls this setting which could be missing or corrupted??