First, check for updates by highlighting the channel/app on the Home Screen, pressing * on your remote, and choosing "Check for updates". If there is one, hopefully it will fix the problem.
If that doesn't work, try removing the channel/app by highlighting it on the Home Screen, pressing * on your remote, and choosing "Remove channel". Then restart your Roku device from Setting->System or Settings->System->Power (as appropriate), and add the channel again. You should have to sign in again. It's the restart that clears out any potentially bad information the channel has stored, so don't skip that step or do the steps in any other order.
The last thing to try is to restart all of your network equipment (gateway/modem/router) even though other devices are working, and then restart your Roku device.
Hopefully one of these will do the trick.
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