To resolve this issue, please Power cycle Chromecast. To do so, follow the steps below:
1. Remove the Chromecast device from the HDMI slot.
2. Unplug the USB cable or power cable (depending on how the device is powered up).
3. Reconnect the Chromecast and try again.
If that doesn't help, please provide the following information:
1. let me know the chromecast speed by going to settings>Network>Test Chromecast Connection
2. Are you using an Android or iOS tablet?
3. What is the make, model and version of the device?
4. Are you trying to cast via chromecast or Roku?
5. What is file format of the videos?
6. Is the issue impacting all videos or a specific one?
7. What type of internet connection(ISP)?
The more specific you are, the better I will be able to assist you.