When looking at the form after connecting, the ISSN field is blank.
As an example, when I break down the information in the URL it is shows:
So it first gives an rft.issn=0301-1526, then it later sends over blank fields for rft.isbn and rft.esbn. The problem is the service provider should only be sending over fields with information in them. When I look at my OpenURLMapping table, it shows the OpenURLFieldValues starting with <#rft.isbn>. So it is looking for that value to put in first. If that value does not exist, then it will put in <#rft.issn>. Even though you have a value for rft.issn, because the rft.isbn is blank, then the information being put in for the ISSN field is blank. You could change the order in the OpenURLMapping table so the <#rft.issn> field is first, but that might cause an issue with another service provider if they send a blank field with <#rftissn>.
If you cannot get them to stop sending over fields with blank values, you will have to change the field Order in the OpenURLMapping table.
Another possibility is that the field is not in the OpenURL Mapping table. Make sure the field that is missing which in this case is ISBN is set up in the OpenURLMapping table.