Flyback Start-up problem
Seems like flyback action cannot be initiated and/or completed.

Remember how flyback converters work: Energy is stored in the primary then is transferred to the "load".

No load, no transfer, no continuous flyback action.

The solution is simple: Put a "dummy load". I personally have no idea what value it should be. But, forcing the converter to work under %2-5 (or even %10) would be enough. For example, your converter's nominal output is 5V/5A. So, placing a resistor to draw at least 0.25A under no load can be enough: 5V/0.25A = 20R/2W.