Answer
Custom Actions only support methods that take simple parameter types (VARCHAR2, NUMBER, DATE) or no parameters at all. A method won't be selectable if it falls into any of these unsupported categories: it returns data (a function, or a procedure with OUT parameters), it uses complex data types like PL/SQL Record Types, it takes BLOB or CLOB parameters, or it's actually a method exposed through a Projection's own service package (_SVC), a client metadata package (_CPI), or a Report package (_RPI) rather than a genuine standalone custom API method. Check whether your specific method falls into one of these categories — that's almost always why an otherwise-valid custom method doesn't show up as a Custom Action candidate.