While we offer the ad space for the videos, it is up to the ads video provider to make ads available to display - occasionally video providers run out of content to display on your specific device model. Different amounts of ads will be available depending on several factors such as device model, operating system version, and location. Older devices will also typically have fewer ads available than newer devices.
When content is made available from the ads video provider for your specific device in your area, you will be able to watch the videos.
There is a possibility that your device also is having an issue displaying the ads. This could be a simple issue of a connection problem with the internet or your device not having enough internal resources to display ads correctly. The best way to check would be simply restarting the app, or even better your entire device.