Sometimes this can be caused by the source material. MP3 audio that is encoded with variable bit rate is often times responsible for poor audio/vido sync when you convert to mpg2 found in dvd-video.
Examine the original files and see what type of audio is used in them. MediaInfo is a good, free program for this. I find the Tree view within MediaInfo is the most informative.
If you find MP3 vbr, you can run it through AviDemux to change the encoding to cbr (constant bit rate). Just copy the video stream, but reencode the audio to MP3 cbr at a bitrate close to the original audio bitrate.
Then import the result into DVDFlick.
The other option is to use AVStoDVD instead of DVDFlick for these problematic files. I find it to be a slightly better program all round, but I'm biased in favor of the HC encoder that you can use in it. HC is only for video though, I believe both DVDFlick and AVStoDVD rely on ffmpeg and Aften for audio encoding.