[ms office - 2013] Access - Combo Box not populating table

[CHR1S]

I have two identical tables (A & B), populated by two identical forms.

 

A) I select an asset category from a combobox populated from another table (two columns, one bound) click save and it populates the table with the selection.

 

B) I select an asset category from a combobox populated from another table (two columns, one bound) click save and it errors with "You must enter a value in 'Assets.AssetCategory' field.

 

Both use the following to populate -

 

SELECT DISTINCTROW [Asset Categories].*
FROM [Asset Categories]
ORDER BY [Asset Categories].AssetCategory;

 

Why would one work and the other not?

[CHR1S]

If I change the Control source of table B's combobox to the non-linked source it works, but it changes the wrong info.

 

I realise my table has a few quirks, like the primary key and its secondary value in the same table instead of it being joined.

 

work to do.

[CHR1S]

So, why would it refuse to populate the correct entry in the table?

I need it to work as its breaking other things.