You'll need to describe the tables, fields and relationships involved. I can guess that you probably have these basic relationships:
But what match fields are specified and which field in the portal is formatted with your drop down list and whether this field is a text or number field are all important details you have yet to share with the forum.
I'm probaly not going to be able to give you all the info you want as I'm not too sure where to find it all.
I've attached a screen grab as a starting point and maybe you would be so kind to direct me on from there. At least it may narrow down what I need to find.
That rules out a few possible causes.
You need to open up Manage | Database and check a few more things:
In Relationships of that window, what are the match fields that link Estimates to Estimate Line items?
On the fields tab, what data type have you specified for Job Number in the Estimate Line Items table?
Are there any auto-enter field options specified for this field? (Double click it in Manage | database | Fields and check what is selected on the auto-enter tab.)
Attached are the screen grabs that may answer your questions.
I see two different issues here.
The one that appears to be the source of your problem is the looked up value setting for the Job Number field in Estimate Line Items. Remove this setting. Since you are using a drop down list to select the job number, you don't also want the same exact field to look up data from another table.
On a lesser note, you give job number examples such as SRX12345, a "number" that includes both text and a number. I recommend that your job numbers be strict serial numbers and that any text be kept in a different field. a calculation field or a pair of merge fields located side by side can display the combined text and number value. If this is a text and number ID generated from a source external to FileMaker, it should be kept at a text field and an internally generated serial number field should be used in your relationships.
Thankyou for both your help and advice. Your reccomendations have solved the problem (you probably knew that anyway) and has helped me out no end.
Once again, many thanks.