I have a Roku Ultra and Express, the latest of several Roku players I have owned. I have never experienced any issues like this before. Both players have started to buffer in the past 3 or 4 weeks, getting worse during that period. I have only had an Amazon Prime account since Oct 2020, and it worked fine until recently. At the same time, I have experienced similar issues with my Britbox account, which has previously worked fine for nearly 2 years. Now, Programs take a long time to load, often resulting in the insufficient bandwidth error. Once playing there is constant buffering, making it unwatchable. I have tried Rebooting, Reloading, Restarting the Roku, my router and my Spectrum modem many times each without effect.
My internet speed is 200Mb, the Roku Ultra runs on the 5K channel of my router at 25Mb and 45Mb when wired. This should be plenty of bandwidth for smooth streaming.
Other apps, YouTube, Roku channel, Netflix work fine.
The link to Amazon support provided earlier in this topic by a Roku moderator links only to the Amazon customer support page, no real help there.
I called Amazon support for this and was able to get them to submit a ticket for it. They seemed surprised about this. It is definitely an issue with the Roku Amazon app and not a network issue for me. As I said earlier, my Android TV app on the same network works great.
I think they have an issue with a recent update to the Roku app as it did not always do this.
I have dual band WiFi, 2.4 or 5.0g. Having this same frequent buffering problem, obtaining a new Roku Streaming Stick+ allowed me to sign on my Roku to my 5.0g Wifi. So far, problem solved. Note that my 3 yr old Roku Premiere Plus did not even show I had the 5.0g WifFi, but in contrast, it showed right up when I set-up the new Roku. Now buffering is minimal and when it does happen, it’s very brief.
Hope this helps.