To harvest seeds, you need to select which plant they came from, and the plant must still be growing.
If you don't have that strain of plants in the 'growing' stage any longer, the workaround is as follows.
From the Grow Overview, click New to create some new seeds and just choose whatever plant you feel like as the mother and create them. Next go into your inventory overview, and click the special View button to see "Seeds".
Then, select the new seeds you made and use Modify>Product to select a Seed template and set the strain. (You might have to create a new Product Template for "Seed" first if you haven't had to do this before.)