lip sync

hi

i am a begginer.I have an issue with lip sync. i used 2 characters and a background. both characters audio in one audio clip. i used same audio clip in few time1.for main timelime 2.both characters body part timeline 3. both characters head inside timeline. every timeline audio started 1st frame. I created auto lipsync for both characters. my issue is, when I see head inside timeline audio with lip sync is perfectly work. but when I back to main scene and test lip sync is not match with audio. there is a big timing issue. if anyone can give me a advive. please..