Did you already try to restart the PRTG Core Server service? If that doesn't help, please pause the Sensors and use our Netflow/IPFix Tester (https://www.paessler.com/tools/netflowtester) to verify that packets are indeed received.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.