Headphone plugged but still no sound on my AP-45

[Dondda]

Hello, 

 

I write here hoping I can get some help on my CASIO AP-45.

It works fine but when I plug in the headphone via 6.5mm jack connector, nothing happens. The sound still comes out of speaker.

According to the user manual, all I need to do is to plug in and the headphone will work. 

 

I tried different jack adapters (from 3.5 to 6.5mm), and open up the keyboard cover to clean the dust. It remains the same.

 

Has anyone encountered the similar issue? Any idea (I'm pretty desparate here....) ? Thank you!