The player one slot is having trouble with controllers. I have to wiggle the plug around in the port for a while before it recognizes the controller. Anyway to fix this. I have a PS Slim BTW.
I don’t know how to fix your problem directly, but be careful when wiggling the controller plug. I also have a slim PS2, and one of the pins in the controller plug mysteriously broke off a while ago and the controller doesn’t work at all anymore.
Try another controller in the port; maybe the problem is with the controller itself, not the console.