Two reasons why the updates you made on your webmix are not showing up on the webmix preview URL:
1. You didn't click Update webmix in the Share menu of your webmix.
2. The updates you made or tiles you added are outside the 10x6 grid. The preview (e.g. http://www.symbaloo.com/mix/tuesdaytutorial) only displays the inner 10x6 tiles of your webmix. Once a user adds your webmix to their account, the complete webmix as well as the wallpaper will be displayed.
For more on how to update your webmix, watch this video: