The reason for this is most likely because you may have copied and pasted the URL for a product page into your news feed, rather than sharing directly from the product page by using the social buttons for sharing.
Even if you have enough points, if the task checklist is not complete, the system will not upgrade you to the next tier.
But, the solution is easy! Make sure you are logged into your account prior to doing this (if you don't have one yet, create an account here):
Just visit a product page, and then click the 'like' and 'share' button (or use the other social buttons if you prefer, see the attached screenshot below) and you can then complete the task. Once that is done, refresh your browser and the task will be marked as complete, and an email will be sent to the inbox you are registered under with the 20% off code.