Answer
Not every Logical Unit or field qualifies for Subscription. At the LU level, it needs a real base table, with OBJID/OBJKEY enabled. At the field level, it needs to exist in both the base table and the view actually used by the client form (client-side derived/calculated attributes don't qualify), needs valid core Foundation properties, and must be one of a limited set of data types — NUMBER, DATE, or VARCHAR2; CLOB columns aren't supported. If a field is missing from the subscribable list, it's almost always because it fails one of these requirements — most commonly being a derived/calculated attribute rather than a real table column.