Internet Download speed

I used to get 30 Mbps download after I upgraded to Cox recommended DOCSIS 3.1 modem, however recently (last month or so) the download speed has been capped at 10 Mbps.  The only reason I upgraded from  DOCSIS 3.0 modem to  DOCSIS 3.1 modem was at Cox suggestion to get 500 Mbps Dn and 30 Mbps Up.  What's the deal!