One possibility is that you have installed AdBlock (including AdBlock Plus and AdBlock Pro) on your computer. PBS videos will not play if you are blocking the ads. To fix this, you can:
turn off AdBlock
- exclude PBS.org pages from being affected by AdBlock
How to fix it
- To turn off AdBlock click the menu in the upper right side of the page and click Pause AdBlock (1).
To exclude pages on PBS.org click the menu in the upper right side and click either Don't run on this page or Don't run on pages on this domain (2).