Trouble creating ISO files from only certain DVDs

I’m in the middle of trying to digitally convert all my DVDs and I’ve run across a curious issue – when creating ISO files, certain discs won’t create the ISO file. I want to maintain the perfect 1:1 copy with menus, chapter selection, etc. so I don’t want to rip to another format (which DOES work just fine on these discs). The DVDs play fine in both a DVD player and also in the drive I’m using to create ISO files – but about 1 out of every 12 just will not create an ISO file. When I attempt to create the ISO file it instantly pops up the “completed” box instead of going through the 15min or so process of creating the ISO. There’s no error message or anything. All discs are region 1. I’m currently using Wonderfox to create the ISO files but I tried using Imgburn and that wouldn’t create an ISO either. Bizarre? I haven’t been able to find anything online about this phenomenon either. Any thoughts? (I haven’t started working on Blu-ray discs yet. these are all dvds so far)