How can I refresh a subform after a combobox item is selected on my main form in access?

I have a subform that displays a bunch of dimensions for whichever part is selected from a dropdown on the main report..

Their are two dropdowns in the main report; JobNum and PartNum.. PartNum is tiered from JobNum so that when you select a job only parts from the selected job are listed in the PartNum combobox.

So I then have a subform that displays all the dimensions associated to that part that is selected. It will update only if I hit the F5 key after I select the Part in that combobox.

What can I do to make that subform automatically update after I select the part in the part combobox?

Thanks in advance for the help!