iPod Nano 5th Gen dock malfunction

I have a external speaker dock I used to connect my iPhone 4S, recently, I found my old iPod Nano 5th Gen and tried to plug it into the dock. It used to work just fine. However, in the last few days, the speaker stopped to play in the middle of the song, and when I hit play on the iPod, its internal speakers started to play the song, and it won't go back to the dock speaker. The only way for it to go back to the speaker sound is by disconnecting and connecting the iPod all over. This can't be a dock issue, it works just fine with the iPhone. I've already tried to reset the settings in the iPod, but this issue keeps coming back. I have no idea how to work this out. 😢