ConvertXtoDVD generally does a good job of converting various types of video files to dvd-video format.
Do you test the output on the computer before trying to burn to a disk? Have it output to a folder on the hard drive, then play the movie using VLC or Media Player Classic HomeCinema (both are free to use). Or you can use your normal player for dvd video on the computer.
You can also try opening the dvd video on the hard drive with DVDShrink. I'm not saying you need to compress anything, just try to open it with Shrink. DVDShrink is stickler for correct dvd structure, and if it cannot open the files, then you know they have not been made properly. DVDShrink can be downloaded from www.mrbass.org
You can also examine the dvd files with MediaInfo and post the information it shows here in your thread. We should be able to spot any problems in the encoding results. I find the Tree view within MediaInfo to be the most informative. MediaInfo is free to use and easily found through Google.
So, that should cover the encoding side of things. Most likely that is not the problem. Next up are the type of disks you are using and the burning speed you have been using. We normally recommend Verbatim as a safe choice for media, but that situation is a bit fluid right now. Use DVDIdentifier to find the mid code of the disks you are using and tell us what that is. A mid code is the manufacturer id code. If you are burning at top speed, reduce that speed somewhat and see if that helps.
It would be good to know the make and model burner you are using also. Do you have the latest firmware for the burner?