Hi Katy:
I'm not sure what's causing that and sounds like you've checked the right things. The field must be enabled (box checked in the "Default Fields" section) and the Advanced Settings box for "Show on insert form when updating a single row of data" must be checked.

As you know, for default fields in a Sample Type, you reach that advanced settings box via the schema browser. Did you check both the samples schema and the exp.materials schema? Check the settings for that field of your sample type in both those places and see if the box is unchecked in either. It seems from my quick check that the samples schema will override whatever is set in exp.materials, though it's worth checking both on your version. Also check both places to be sure that there is no XML metadata overriding the checkbox with <shownInInsertView>false</shownInInsertView>.

Hope that helps,
--Molly