Problem with Bluetooth mode

[NSch1]

Hello and sorry for my bad english.
I have a family Account and today i paired my samsung s4 mini with my Bluetooth car radio. The music is playing and every thing is good until my phone gets the locking screen. The music is still continueing until the end of the Song but then its not changeing to the next one. But when i unlock the screen, the next song starts to play. What can i do?