Can't your conversation partner see you?
Check whether you have authorised the app to use your camera. You can find this in the settings of your device in the app permissions section.
Can't see your conversation partner?
Have your conversational partner check whether he/she has authorised the app to use the camera.
Still not working?
Get as close as possible to your Wi-Fi modem so that your Wi-Fi signal is as strong as possible. Also try connecting your device to 4G if image calling fails via wifi, but be aware that image calling on 4G consumes a lot of data. Ask your conversation partner to do the same.
Still doesn't work after that?
Log out of the app, close the app, delete the app and reinstall the app. Try video calling again. Ask the person you are talking to to do the same.