Value in Drop down list disappears
 

I created a spreadsheet with a drop down list to populate a cell. I got this
message from the user:

At times there is a problem with the attachment materials cost on the
Materials page. The user must select from the drop down list which value to
enter (Standard, Special or Erie). The problem is that sometimes (I have not
been able to figure out why it is only sometimes and not consistent) the
value in the drop down box does not stay permanent. If the user opens a
file, these values may or may not be populated.

Does anyone know why this might happen?

Thanks.