It can happen if your internet connection is unstable. Please make sure that you're connected to Wi-Fi and have allowed your device to download data. After that force close Prequel and start it again.
If you need any assistance, feel free to contact us! You can do it here in the app via Prequel's settings menu (Send feedback option) or at support@prequelapp.com.
Please, describe the situation as detailed as possible: what actions lead to the issue, how often does it happen, screen records of the issue, etc.