It is better to provide too much detail than not enough. We can always skiim over the extra detail, but have to ask you to provide the missing details before we can then offer a solution...
I think this is the heart of the matter:
I do have a serialized ID for each check in item but at this point it is not used for anything other than a placeholder.
This is field is your primary key and should be used to link a record in your instrument table to records in related tables. Since you aren't using that field, you already are set up for problems here as other fields used for this purpose can be very problematic.
I can't suggest much beyond that without knowing more about your databse. Try listing the tables you use for instrument repair and describe how they are related to each other. You may also need to describe the design of your layout.
Here's a simple format for describing the relationships you've set up in Manage | database | relationships:
Instruments::__pk_InstrumentID = Repairs::_fk_InstrumentID
Instruments and Repairs would be two of the "boxes" on the relationships tab. __pk_InstrumentID would be an auto-entered serial number and _fk_InsturmentId would be a number field and these two fields are used as match fields in this relationship. (__pk is a naming convention you can use to identify a "primary key" and _fk is a convention for identifying a "foreign key".)
I will try to list more detail soon.