Verify that the NORDIC_BLE protocol is enabled in Wireshark by clicking Analyze > Enabled Protocols and checking that NORDIC_BLE is selected.
Ensure that you are using a stable release of Wireshark. Development and user build versions are not supported. For example, v3.0.7 and v3.2.0 are stable versions of Wireshark, as indicated by the second number being an even number. Version 3.1.x is a development version of Wireshark, as indicated by the second number being an odd number.