Missing Something

I hope someone can help me with this. I figure I'm missing one line of code but can't figure it out. I have 3 tables - tblGeneral (which is my main table), tblMDLink, and tblMD. The primary key in tbl General is FileNo. The primary key in tblMDLink is an autonumber and the primary key in tblMD is also an autonumber called MDID. tblMDLink is just a linking table containing the fields Fileno and MDID and uses an autonumber as its primary key. It contains some other fields that have nothing to do with this. I have a query called qryMDLink which contains all the fields of tblMDLink together with a calculated field calle full name which just combines the last name and first name fields of tblMD. Now I come to my form (which is actually a subfrom) called subfrmIMEDoctor. The purpose of the fomr is to allow users to add the names of various doctors to various files. The record source of the form is qryMDLink. The form also contains a FileNo field which is added programatically. The field has a drop down combo box with its record souce set to MDID. The drop down box has two columns with the first (not visible to the user) set to MDID and the second set to Full Name. Here's the problem - it won't let me enter data. Whenver I click on the box I get the message - "You cannot add or change a record because a related record is required in table tblIMEDoctor". By the way, I did set up one to many relationships beteen tblGeneral & tblIMELink and between tblIMEDoctor and tblIMELink. Any help would be greatly appresciated. Thanks,
Tom Gorton