Here's a list of common things to check.

Are you using the wrong slave mode?

For your speedlight to be a radio slave, the antenna icon must be showing, and the backlight should be orange.

Godox's TT685 and V860II speedlights for Canon, Nikon, and Sony have five different syncing modes that the horizontal lightning bolt button cycles you through:

• On-camera mode (no wireless): no icon, green backlight
• "smart" optical master: lightning bolt icon, green backlight
• "smart" optical slave: lightning bolt icon, orange backlight
• radio master: antenna icon, green backlight
• radio slave: antenna icon, orange backlight

The V1 and the TT685/V860II for Fuji and Olympus only have three modes:

• On-camera mode (no wireless): no icon, green backlight
• radio master: antenna icon, green backlight
• radio slave: antenna icon, orange backlight

It's easy to confuse the optical and radio modes, or to mistake the on-camera mode for radio master. Pay attention to the absence/presence of the sync mode icon on the LCD, as well as the color of the backlight.

On the TT350/V350 mini speedlights the backlight is always orange, there are no "smart" optical modes, and the antenna icon and S/M indicator is in the lower left corner of the LCD. Swapping modes is done with the SYNC button and dial. Confusingly, the SLAVE button the TT350 is only good for the "dumb" S1/S2 modes when the flash is set for on-camera use in M mode.

Are you using the flash and transmitter close together?

There is a bug in Godox's radio gear that prevents a radio connection if the transmitter and receiver units are used within one meter of each other. You have to set "close mode" on the transmitter to achieve sync.

For more details, see: How do I get my Godox trigger and flash to work close to each other?

Are you using silent/electronic shutter?

With mirrorless cameras, electronic shutter modes often disable flash. This is because the way that line-by-line readout is done lowers the sync speed to very slow values (e.g., 1/15s). To enable the flash, turn off silent or electronic shutter modes.

EFCS (electronic first/front curtain shutter) won't disable flash, but may reduce your sync speed or cause banding with HSS.

For more details on how electronic shutter affects sync speed see: dpreview's "Electronic shutter, rolling shutter and flash: what you need to know".

Do the ID code settings match?

With the XPro transmitter, Godox introduced a feature called "wireless ID". This is basically their implementation of the old PocketWizard Custom ID, where an additional digital code is used to filter out signals from other nearby trigger users on the same channel and prevent them from popping off your lights.

With newer Godox triggers, the ID is set with the MENU button and setting the ID function. 00 turns off the feature. 01 to 99 are the possible values for the ID code, and the code must be identically set on all units for the system to work.

All the Godox speedlights (except the TT600 and V850II), receivers, and transmitters can use ID codes, but may require a firmware update to have the feature. With the TT600/V850II, the ID must be set to 00 (off) for the flashes to trigger.

See also: How do I tell if my Godox trigger or flash needs a firmware update?

Is your camera's flash mode set properly?

Check that you don't have flash disabled in your camera menus.

Also, at this time, AFAIK, the only camera menu system that has full two-way communication with Godox gear is Canon. It's not uncommon with non-Canon gear for flash control mode settings to be grayed out when a Godox flash or transmitter is on the hotshoe.

On all other systems, despite the fact that you're using a wireless flash, you don't want to use your camera's menus for wireless flash settings, which are for their proprietary optical wireless system. You pretty much want the camera to think the transmitter on the hotshoe is just an OEM speedlight.

With Sony cameras, it's also recommended that you set the flash mode on the camera to "Fill Flash" to get Godox gear to work properly.

Did you try a different channel?

Godox gear operates on the 2.4 GHz band, which is where nearly everything else wireless operates. There may be no way around an interference issue if you can't switch your location, but try changing the channel you've chosen.

Must be your wireless id. Your checked everything else inclucing the channel. The one thing everyone forgets is the id#.