Roku TV is consistently unavailable with Cast for Roku

I pull up Cast For Roku on my phone and TV, and the TV shows up in the list but about half the time it says it is unavailable. I'm not changing any settings. I've tried restarting both the TV, the phone, the Wi-Fi connection (to both of them), etc, and nothing seems to help.

Sometimes it works. It seems to be random whether it is available to connect or not. But for most of the last couple of days it has been greyed out, showing up in the list of devices but completely unavailable.