Tail gate won't stay closed

Likely either the tailgate latch cable is rusty/damaged and is sticking, locking the latch open or mostly open, or something has gotten knocked out of alignment so that the latch can't completely shut. Disconnect the cable from the latch and use a screwdriver to trigger the locking mechanism to see how it works, then track down what the problem is - if it locks easily on the screwdriver, try it with the cable attached. If it still locks easily it's an alignment issue with the striker. If it worked with the cable detached but not attached it's the cable. And if it doesn't work either way it's likely a damaged or sticking/rusted latch.