Subform combo box based on combo box

[eddyfuente]

I am creating a database and ran into an issue I do not know how to resolve. I created a datasheet form (frmMSRSubRecords) and based one combo box (cboPartNo) on another(cboCategory). The cboPartNo field is bound to a materials tables (frmMaterialRecords) and automatically updates other fields in the form based on the cboCategory field. This works fine and all fields update and save properly.

This form is meant to be a sub form for another form (frmMaterialServiceRequest). The form is linked by the key field MSRID. When I test cboCategory works fine but when I try to use cboPartNo I get propmted "Forms!frmMSRSubRecords!cboCategory". I enter the Category and it does what is expected. The thing is it should use the data from the cboCategory field automatically instead of prompting for the information (like it does when you open frmMSRSubRecords).

Any assistance is greatly appreciated. I have attached a copy of the database.
Thanks
Eddy