If the products are positioned in a place with poor WIFI signal, this can cause trouble. Dual band WIFI has been known to cause problems, make sure that all your products are on the same Wi-Fi network, either 2.4GHz or 5.0GHz. If your Wi-Fi creates different networks (such as a separate 5g network) it's important to make sure that your speakers are connected to the same one.
Please check that your Bang & Olufsen speaker is enabled for Multiroom through the Bang & Olufsen app. A full list is available here.
If you continue to have problems, we encourage all users to submit feedback through the Feedback box which can be found in Settings in the app. This ensures that the feedback is received by the developers with the necessary technical context.