DIAGNOSIS This is almost always a permissions or device grouping issue. The Hub needs explicit permission to access those camera feeds, and they need to be in the same "Home" within the Alexa ecosystem.
FIX First, in the Alexa app, go to Devices > Cameras. Select your camera (e.g., Ring Doorbell). Tap the gear icon, then tap Camera Privacy Settings. Ensure "Allow In-Home Streaming to Echo Show and Fire TV" is ON. Do this for each camera. Then, on the Hub screen itself, swipe down for settings, go to Home & Widgets, and make sure the camera widget is added and pointing to the correct device.
IF THAT DIDN'T WORK Check your Ring or Blink skill in the Alexa app. Go to More > Skills & Games > Your Skills. Find the Ring or Blink skill and disable it, then re-enable it and re-link your account. This refreshes the authorization token, which can expire or corrupt.
HEADS UP Live streaming to the Hub uses a lot of bandwidth. If your Wi-Fi is congested or the signal to the Hub is weak, the stream will fail to load. A loading spinner that eventually times out is a classic sign of poor network conditions for that device.