When attempting to start a stock comparison, you may find that one of the stocks you want to compare is not available. This can happen if you have not completed the SSG for the stock in question.
To solve this, open the ssg for the company or companies which are not available for comparison. Complete the ssg for any company you wish to make a comparison for, and make sure to save the ssg when you close it. Once the ssg is completed, the company will be available the next time you wish to make a comparison.