USB ports not working on Thunderbolt monitor

I have had my Bose USB speakers connected to one of the USB ports on my 27 inch Thunderbolt monitor since I purchased it in April.

Yesterday the speakers no longer worked, and they don't show up as a sound option in system preferences. Since I use an Elgato Thunderbolt dock in the chain, I plugged the speakers into one of its USB ports and they work fine. I did start using both USB ports on my Macbook Pro yesterday to run a RAID using two Seagate drives for Time Machine.

I am trying to figure out why the USB ports on the monitor no longer appear to be working. Have I reached some limit on the number of USB ports the Mac will recognize in the chain? If so, how come I can still get sound through the Elgato dock?

(I just plugged my iPhone charging cable into the back of the monitor, and there is nothing. So the USB ports on the back of the monitor appear to now be inactive. I did this to confirm it wasn't an issue caused by the Bose speakers.)