Dropdowns not listed as User Defined Fields in Lookup

Dropdowns not listed as User Defined Fields in Lookup

I am a DabbleDB user attempting to rebuild in Zoho.  Your Lookup fields are wonderful, and this is a feature that didn't work very well in DabbleDB (which is why I'm rebuilding from scratch rather than importing).  But I keep running into the same problem:  In Form2, I want to add a Lookup field that refers to a Dropdown field from Form1, but none of the Dropdown fields from Form1 appear in the list of User Defined Fields in the Lookup dialog box.  All the other fields from FormA seem to appear just fine, except the Dropdowns, which are the ones I need.

Two screenshots are attached.

Debbie