DVD FIles & Skip Function

I am not sure that this specific issue has been addressed;

I have numerous DVDs that I have created over time which are collections of shorter videos (music videos etc). ALL of them were created using the same software (Nero),

I have saved several of these to the WD hub as ISO files and while all will play only 1 out of three will allow me to skip from one video chapter to the next. The only way for me to go from one video to the next is to go back to the DVD menu and select the next video. This is very inconvenient as sometimes I want to simply skip to the next video.
 I have looked carefully at how I have compiled and burned the various DVDs and can see no difference yet only one out of three works properly.

 Any suggestions?

Open the DVD files with Handbrake to see if they are actaully separated by CHAPTER or TITLE.

Just to check I burned one of the ISO files in question to disc and then reopened it with DVDShrink and all of the seperate file info was present.