This happen to me, but only one side did not latch.
Sounds like the rods are not adjusted correctly.
When you close the tailgate. do they even latch?
If not the rods are adjusted - short.
You can remove the tailgate handle bezel. Looking at the latch. You will see two rods. They are clamped to the moving parts of the latch by green clam-closing clamp. You can pop these open and re-adjust the length.
What I did on mine was to, with the gate open. Work one latch at a time.
To test if latch is latching.
With a screwdriver, close the locking latch on the of the tailgate. Has if you had closed the gate.
You should hear a click. If not, the rod is short. (Mine just spring back into the open position)
If that's the case, (under the handle) open the green clamp, allowing the rod to be loose.
Close the latch again with the screwdriver. You should hear or feel the click.
Now put the rod back into the green clamp, and close clamp.
Now lift the gate handle, and latch should unlock.
Repeat for otherside.