Since you state 12 flawless backups (once a day?) I am going to assume that you have a retention period of that number.
What happens is, the first backup is a VBK, and then the others are always incremental (VIB). But when the retention period is over, we need to merge the VBK with the oldest VIB. I think it is in that process that things go wrong.
Is the VBK available at the backup location?