Backup Exec 2012: Hyper-V differential backups and fixed disk issue?

Hello all,

just a quick inquiry if I'm on the right track: is it possible that Differential/Incremental Backups won't work (fall back to full) for a Hyper-V-VM which is using fixed disks (fixed size VHDs)?

I'm currently experiencing this issue in my backups. Is there any tweak / workaround for this available, or do I have to live with it for the time being?

Thanks in advance, Manuel