2012 RAV4 Radio on/off knob

I recently purchased a used 2012 RAV4. The radio on/off knob would not turn the radio on and off. After three weeks, the used car dealer replaced the knob this past Friday. The knob worked when I left the car dealer. Yesterday (Saturday) I went to show my wife that it was working, and it would not. It seems like there is no spring behind the knob. When I push it, it stays down and does not pop out. Any reason for what is happening? And any easy fix?:confused: Love the RAV4 but this is becoming a bit irritating.:frown: From a newbie. Thanks in advance. Oh yes, it has 26,000 miles so should still be under warranty.