I've had this issue with Bluetooth audio devices in the past. Sometimes its the music app that is acting up or the audio on the phone that is the issue. But these issues would go away if I plugged in the aux cord or wired headsets.
I found that powering down the phone and restarting it helps. I would try unpairing the speaker. Restart your phone and speaker. Then connect to the speaker again. Try that a couple of times if that doesn't work or if you have to do this method every time I would reach out to firstname.lastname@example.org. It should connect and play music via bluetooth every time.
Make sure to tell them everything you have tried. I've heard that are very busy right now and might take some time to get back to you. But overall they will work with you and make things right.