Sounds like the videos you are trying to convert now are a different format than the previous ones. Are they? AVC won’t work on every format out there. There are other alternative conversion programs; you might have to experiment until you find one that works before turning it over to SMC for final conversion and transfer to your Fuze.
If you post a bit about the ones you were sucessful with, and those that are giving you trouble, perhaps someone here has already conquered that format and can advise you. 
