The app can't find my hub
Nine times out of ten this is a network segmentation problem — phone and hub on different VLANs or one of them on a guest network. See First-time setup → If it doesn't find your hub.
A bulb won't pair
Power-cycle the bulb (off, 5 seconds, on) and try again. If it was previously paired to another hub or app, factory-reset it first — the process is bulb-specific; see the relevant light-hub page.
Lights change on their own
That's the point — but if it feels wrong, the circadian engine
may be set too aggressively. Open Plan → Engine and dial the
melatonin guard down a notch.
Remote access stops working
Remote access goes through api.rhythm.lighting. If your
network blocks outbound HTTPS to that domain (some corporate networks
do), only local control will work.