can't backup nandroid data lollipop on CWM 6.0.5.1? (error mounting /data) topic

hello
why can't I do a nandroid backup of my lollipop rom on CWM 6.0.5.1 succesfully?
I am using the new version of 6.0.5.1 cwm.
it can back up the system and the other thing before (I forgot), but keep getting error on backing up data. it says can't mount /data?

since the backup isn't complete I have to do advanced restore to restore system.
what should I do? replies would greatly appreciated :D

thank you