Where are your music files stored ? If they are on the Macbook Pro then that might explain it.
Remember that with Roon the signal always goes through the Core. Ethernet is recommended for the links between Storage, Core and Output rather than WiFi because Ethernet is full duplex while WiFi is not. So any link where you are trying to send files from Storage to Core and also from Core to Output over the same WiFi link is going to be problematic.