The TV guide is empty, or shows the wrong programs
Four causes in order of likelihood, and how to tell which one you have in under a minute.
An empty guide and a mismatched guide are different problems. Work down this list in order.
1. The guide has not finished downloading
It is the largest file in a sync and it lands last. On a first setup, give it a few minutes and look again before changing anything.
2. There is no guide address
M3U links usually carry no guide at all. Open Settings, then Playlists, then Edit Playlist, and check whether a program listings address is filled in. See Add an XMLTV guide address.
3. Some channels have a guide and some do not
This is a name matching problem, and it is the most common one of all. A program is attached to a channel by name, so a guide that calls a channel Sky Sports Main Event HD will not attach to a channel your list calls |UK| SKY SP MAIN EV.
The Title Optimizer helps here. Stripping tags and country codes from channel names makes far more of them line up with the guide. See Clean up messy channel names.
4. The guide covers other channels entirely
A public guide address built for another country will match almost nothing on your list. The guide that matches your channels is the one your own service publishes. Ask them for it.
Forcing a fresh download
Settings, then Playlists, then Refresh Playlist. If the sync ends with a warning saying the guide could not be downloaded, the address is wrong or the guide server is down.
Related reading: why some channels have no guide.