A named range I have in the workbook is not showing up on the User Interface Designer

There is a maximum number of cells allowed in table named ranges (MaximumCellCount). This limitation is there to improve the performance of your application during design-time.

The total number of cells is limited to 1,000 on Trial accounts and 20,000 on public cloud accounts. These numbers also include the headers. For example, if you have a Table Named Range with  30 columns and 1,000 rows, it will not be available in the User Interface Designer, because the total number of cells exceeds the 20,000 limit.

This is a configurable option in server and private cloud licenses. When logged in as the “systemadmin” user, go to Tools > Designer > Configuration Settings, and press the Edit button next to the MaximumCellCount field to change this value.