Why is my ps2 slim 1st controller port not functioning?

My ps2 slim has a problem where the 1st port (where you plug in you controller) does not function. If I plug in my ps2 controller I notice that the red analog button shines when I press the analog button (on the dualshock 2 controller) but any imputes I place by pressing buttons don’t seem to register. The 2nd (ps2 slim) port works but I can only control it via the playstation 2 menu and no games. what can I do?