Roku +4K Express will not start apps

I have 4 Roku +4K express units. All work fine except for 1. It’s the closest to the Wi-Fi router (15 feet) and is usually on the 5G band. For some reason after it sits for a while and you go to use it the apps won’t start. They just hang and the advertisement on the far right of the screen is always blank. Only way to fix it is reboot the unit and it works every time. Checking the Internet connection on the screen shows an “Excellent” connection. Any ideas ??