I had this same issue, its a glitch, there is a service request to fix it (SPR#: 982930). the work around i was given is to right click the BOM and select save. then change the file type to excel. save it then edit you BOM in excel and then copy and past it back into the drawing. The other option i was given by John Stoltzfus was to use custom properties in the parts themselves to save material information etc then create a custom BOM template with references to your custom properties thus eliminating repetitive data input as well as saving information with your files thus allowing you to pull it up and re-use it whenever.
Thanks Tyson, I thought that might be the case. I have been using the custom properties method which probably works out better in the long run anyways