Microsoft Access Databases must really have primary keys applied to each core table to help manage relationships and keep data integrity 100% enforced to avoid those unexpected errors.
Take a look at this quick tutorial on How To Set the Primary in Microsoft Access
My Comments on Microsoft Access Database Primary Keys
I’m not a fan of mutiple fields to act as a group for the primary key as discussed in this video tutorial. I believe there should only be one field which is unique (primary) and have other fields set as secondary keys (foreign keys) in your Access database.
I use the method as shown and then check the property ‘Indexed‘ in the lower half view (Properties) for the selected field to check whether it’s set correctly i.e. ‘Duplicates OK’ or ‘No Duplicates’.
I have eBook tutorials and detailed explanation and one in particular is recommended should you need to know How to Relate and Understand Relationships with Microsoft Access Datbases. Great value and comes with a money back guarantee too!