Fill port won’t work?

So I disassembled my tank to repair valve. Got it fixed and back together nicely.
Problem is now I can’t fill through the fill port!!! But I can fill it with the adapter that’s meant for standard tanks. I have the spin loc tank.

Why can’t I fill through the fill port? Does it need to be pressurized the first time with a scuba to keep the valve closed after that point?

And if so, why does the air coming in through the adapter not escape from the fill port, but when I try to go through the fill port, the air just comes out of it when I try to unhook the pump?

What’s going on fellers??