On 2/250 plan, 4 DS & 1 OFDM channels no longer locked

Since around Aug. 24 or a few days before that, of 32 DS channels, 4 are no longer locked (CH 29-32). And of the 2 OFDM channels, only 1 is locked. Prior to that, all 32 DS and 2 OFDM channels were locked and I was getting the correct speeds. Since this happened my download speeds have reverted back to about 1.2 - 1.3 gbps. US has not been affected. How can this be resolved as I'm no longer getting the bandwidth I'm signed up for? And what may have caused this change? The modem is a Netgear CM3000.