5th wheel receiver hitch latch not completly closing

What type of hitch? Most of the time if the hitch does not latch it is due to the jaws not being complety engaged or in a bind.
If you are connecting up, I'd reverse what you've done, pull away and start over. You may be high hitched or off side to side.
When you connect, are you following the manufacturer's instructions? Most have you starting off with the jaws open and the kingpin about an inch below the platform?
Jim