Suddenly, Subform Flips Me Off

this never used to be an issue, but all of the sudden it just started happening.

Setup:
frmCustomers (data: tblCustomers) is linked to sfrmJobs (data: tblJobs) via CustomerID (autonumber). Depending on their [Status] in frmCustomers (dropdown, either "Customer", "Lead", "Lead-to-customer" or null), a customer may or may not have a job in tblJobs (or may have more than one).

Issue:
When scrolling through my list of customers , after passing by a "Lead" (or if i just select a "Lead" to begin with), who cannot and does not have a job, the subform does not show any jobs from that point on for people who do have jobs. Also when I create a new job and then attempt to insert data, it tells me that I have an invalid use of null, which is weird because the only field that needs to exist is the customerID, and that's created automatically.

Diagnosis:
I think it has something to do with the CustomerID not functioning correctly, which i don't understand at all, or somehow not getting passed to the subform. i've entered plenty of new customers into the program without an issue, and i used to be able to scroll through all of them and have all of them show up all the time. Additionally, even after i clicked away all the "index or primary key cannot contain a null value" errors, it still gave me a CustomerID, but after I created a job (again, "index or pk blah blah") for a customer, i looked at the table and it showed that the JobID (primary key of tblJobs, is not autonumber) was recorded, but not the customerID.

Cry For Help:
Please help me! I discovered this bug almost 5 minutes after publicly deploying my program to my client's company, and if he finds it before i can fix it, i'm toast. please please help!

Thanks to all in advance!

-Jason