Hey Hija,
It's highly likely the clip that connects to the actuator in the gate has broken. This is a common issue with these tailgates. I don't own my discovery anymore but I did source up a video of another guy troubleshooting the same problem.
[link|https://www.youtube.com/watch?v=fo_bw0_qzU8|LR3 Tailgate issue Video|new_window=true]
Its a good video showing you how you can replace the clip.
Cheers!!
Good luck with your repair.
Moe