You're probably converting using a too high bitrate. Even though your DVDR discs state that they can hold 2 hours of video, their maximum size is limited by the amount of available space (and thus, the size of the files is the leading factor here, not the length of the movies). So, if your 30 minute video, after converting, is 3.5GB in size, you can only fit one on a DVDR disc (4.4GB free space). I'm no expert when it comes to converting files so I can't help you with that but if you post more details on which software you're using to convert the files, maybe someone else can help you with that. Good luck!