It happened to me only once and it was due to settings mismatch. All i did is click on " settings for standard mavlink" on both units and it worked. I never modified any settings after that as i only were using it for remote access. Check below my Sik radio settings. Works perfectly (pic is only for my one unit just to show you).
One suggestion: when you make changes in unit don’t copy to remote, connect( through USB) both unit individual and save the config, this way you will have the confirmation that both units has same config, because copy to remote some time config only save on USB connected device not on remote device.
