Passenger door won't lock

I've got a 2003 Dodge Ram 1500. I been having this problem for a while
but never really decided to fix or look into it, but I've become pretty
annoyed by it now since I'm starting to invest more money into my truck.

The passenger door won't lock, it's always stuck in "unlock". I've got a
manual lock so you've got to actually push down the button thing, whatever
it is. Would really be helpful if someone could help me find a solution to this
as I rather avoid going to a mechanic and having to pay a ridiculous
amount to fix this problem as it only seems to be not that big of a problem.

Side note, I installed a remote door lock a while back, had no problems
whatsoever. Think this might be something that could be causing the
problem?

Thanks in advance.