Each tour is streamed via adaptive bitrate streaming. That's just a fancy way to say that the video quality will automatically adjust to fit the bandwidth you have available. Still, there is a minimum required bandwidth and if your internet speed dips below that you will see 'Buffering...' displayed on the screen.
Things to try:
Ensure you're using wifi: If you're on a phone, you may want to ensure you're on wifi and not cellular.
Test available wifi bandwidth: Place your phone or tablet on the machine as normal and load fast.com on your mobile web browser. This will run a speed test. After a few seconds you should have a result. In general 6 Mbps or higher is a good speed to have, while 2Mbps should be okay for lower quality streaming.
If you have less than "megabit" may show a "Kbps" or "kilobit per second" result which indicates your connection isn't currently fast enough for reliably streaming.
If you have a slow result, try moving very close to your wifi router and running the test again. If it's significantly faster this indicates a problem with your local wifi link when you are in the exercise location. Some users have even reported the problem is their exercise machine has a metal frame which blocks the wifi signal.
Download tours locally: You can always download tours locally to the device so you can use them offline at full quality. Before starting any tour you can instead tap the "download" button in the top right. When it reaches 100% you can press Start and instead of streaming the tour you will play it back from local storage.
If you're seeing another message or behavior, please let me know precisely what you're seeing via a support ticket and we'll go from there.