Why does the Roku app sometimes detect the device but ignore most commands?

Yeah that exact scenario has hit me a bunch too—app sees the Roku no problem but then most buttons do nothing, or maybe just the volume works while everything else ghosts you. In my case it often boils down to the phone and Roku not really syncing properly over the network even though they're on the same Wi-Fi; sometimes a quick restart of both fixes it temporarily, or turning off any VPN helps because that messes with the signal. I've also noticed it happens more when the app hasn't been updated in a while. If you're still stuck though, this thread I came across has some solid personal takes on it: Roku Remote App Not Working. Not saying it's a magic fix, just what worked for a few folks I know.