No sound and sound bar on iPhone 5 connected via lightning to 30 pin adapter to speaker dock.

I have a quite annoying problem with my iPhone 5. When I am connecting it to my Sony V20ip speaker dock via Apple Lightnig to 30 pin adapter sound bar disappears and speaker gives no sound. Sometimes it works by several minutes but then music fails again. Charging works perfectly fine. I don't know what is wrong. Maybe it is adapter or iPhone's lightning port? Maybe there is someone who has or had such problem and solved it somehow? Thanks for any help ;)