Docsis3.1 modem no longer connecting high speed upstream

I have an Arris SurfBoard SB8200 cable modem that has been connecting with Docsis3.1 mode for upstream and downstream channels.  I recently noticed that the upstream channel is no longer connecting with 3.1 and has downgraded to 3.0   (downstream channel is connecting at 3.1).  Is there a reason why the upstream channel no longer takes advantage of the 3.1 capabilities?