Check their immediate relay if that relay have those download file cached or not -
*If not, check their parent relay & so on to find out which relay was not able to cache the downloads.
*If yes, then try to validate if there is any bandwidth issue in between this relay & clients where download stuck.
You can also look into action info for more details what is waiting for download, Is there any specific patch or task which is creating problem some where or there is something else.
Most importantly -
–> I believe UDP communication is open on all endpoints & if not command polling is properly configured.
–> All endpoints are Online.
There is another similar discussion, might help -