ImgBurn burns what DVD Flick tells it to, and if there was a problem with the burning process, then ImgBurn would have told you. If there was no problem shown then I guess DVD Flick didn't encode the whole thing. Are you using the latest version? Have you looked in their forum to see if other users had the same problem?
By the way, the 120min shown on DVDs/cases is just a reference: you can burn less than that with more picture quality, or more than than that amount but with a really bad picture quality