There aren't very many jobs where all you do is program PLCs. People who write PLC programs are generally responsible for most, if not all of the controls on a machine or process. The PLC is just one of many tools in your toolbox.
To land a position where you get to write PLC programs, you need to demonstrate a thorough knowlege of the things the PLC is going to be controlling (the outputs) and the things the PLC monitors (the inputs) in order to make the decisions about how to control them. Learn all you can about the components in use in the industries you expect to be working in.
Take a walk around your facility. Anything with wires connected to it is probably involved the control system. If you don't recognize it or understand what it does or how it works, find out.
In addition, learn enough of the science involved in getting the job done at your facility. That would be enough physics so that if the boss says you have to move 1000 pounds from point A to point B in half a second, you can figure out how big a pneumatic or hydraulic cylinder it will take to make it happen. If you work in the process industry, learn enough chemistry to understand the processes your PLC program will be controlling. If you're controlling thermal processes, you'll need a basic understanding of thermodynamics. No matter what the industry, you'll need enough math to be able to understand the equations that establish the relationshps between the parameters.
