In order to prevent freezing up of the playout engine, if a track takes longer than 10 seconds to load, it will timeout and PlayIt Live will consider it to have failed to load. If all your audio files are on the same computer and they are taking more than 10 seconds to load, you should investigate why your storage device (hard drive, solid-state drive) is so slow. This can be caused by an ageing drive, a heavily fragmented hard drive, or other 'junk' installed on the computer that are taking up resources.
If your audio files are stored on a network drive over a slow network, consider investigating why the network is slow. Ensure your computer and network storage devices are connected directly to your router or access point via ethernet.
If all else fails, you can override the load timeout. You can do this by going to File > Settings > Advanced.
The settings box should look like this: