Backflush Handling Unit Error

Hi,

When trying to put material into a backflush handling unit from an IM location using ZWX46, I get an error message that reads "Material move completed.  Tag Merge to Backflush HU failed.

Instead of putting this material into the correct HU, it puts it into a new HU - this is where I try to use ZWX18A and perform a backflush tag merge, function 4500.  I then get an error message that says "Call of transaction LT01 failed for Material XXXX Message no. ZD131."

I cannot seem to understand why I am unable to put material into this backflush bin.  Any help would be greatly appreciated.