If you've created a Live Feed from a Custom Report with a large amount of data, it may not work.
Because the data needs to have loaded in your Custom Report before it can be pulled through to the Live Feed, you'll need to edit your report to be smaller.
Follow the instructions in Step 2 here: Edit the report content
Once you've made a smaller report, you'll be able to create your Live Feed from this data using the Download button. Follow this guide on Using Live Feeds to see how to set this up. You'll need to do this for each report copy to bring in all of your data into your Live Feed.