Couple of things
Try putting BITS into foreground mode
https://www.ajtek.ca/wsus/wsus-bits-foreground-priority-mode-vs-background-priority-mode/
If that doesn't work, try forcing a stop of downloads, declining the updates that need to be downloaded, run the server cleanup wizard (SCW), and then go re-approve those updates.
(Get-WsusServer).CancelAllDownloads()