When i put my key in my driver door to lock the car, the passenger side lock stays unlocked. What would cause it? When I go to the passenger side and open the door and lock it and shut it, the lock pops up. Any help on to why it wouldn't lock? thanks

 

What year???
Most likely a bad lock actuator.
There is tons of posting on this site, may want to start reading some of them.

 

Exact same problem on my 2003 DII a few months back around 50K, it was a bad power door lock actuator that was covered under an after-market, extended, warranty.