Another potential cause for not being able to bind is having a different radio firmware on the receiver vs the transmitter module. I recently ran into this issue with some of my setups. The RX was on 2.X.X firmware, which FrSky enabled some minor attempts at encryption to discourage unauthorized receivers and transmitters. This firmware is NOT backward compatible with the 1.X.X radio firmware, so if your internal module on your transmitter is not updated to 2.X.X firmwares it WON'T bind. You will need to upgrade your internal radio module to the correct firmware as well. You can find the current 2.X.X firmwares here if needed. Keep in mind if you upgrade you'll need to upgrade ALL your RXs in all your quads, and you won't be able to go back unless you downgrade ALL your RXs and radio firmware again.
Also, it's worth pointing out that for all their struggles, within a few weeks there was an update available for the open-source multi-protocol modules that allows binding to the 2.X.X firmware RXs. So, in the end, they've caused a lot of inconvenience for users and not accomplished their goals. Make of that what you will.