Problem with retention date on disk backup after a tape backup

Hello,

we have a DPM 2010 server for backup a cluster Hyper-V 2008.

all VMs are backuped on disk with a retention of 20 days.

all saturday, we backup certain data of this VM on a tape. 

after  a backup on a tape, we can't restore data stored on disk because we can't see date backup on the scheduler of restore.

Thanks for your help