Check the disks - are you using the same brand & quality now as you did when you had the successful burns? (I have had some - not many - problems with lower quality disks)
I have tried several differnt DVD mfg. I am using IDVD^ on a new G%. I simply drag imovie onto time line and burn. It will play back on the Imac but when placed in a DVD player I get a "not formatted for repaly " error