Auto Lookup In Query Not Updating Underlying Table

I have created a 'junction' table between 2 many to many tables:

Jobs table (details all jobs)Jobparts table ('junction' table details Job ID, Part ID, quantity and unit price)Stock table (details all stock).The fields Job ID, Part ID and Unit price are each lookup fields from the corresponding fields in the jobs and Stock tables with quantity being a manual entry. This works fine.

I discovered you can automatically populate the unit price field using autolookup based on the entry of the Part ID field on the form. However having amended some pre-existing jobpart details randomly on the form when I checked back to the table they had not updated. Similarly when creating new entries while the form 'unit price automatically updated on entry of the 'Part ID' these details did not feed through into the table.

I assume it has to do with how the table was originally set up the field properties will need to be changed.