Can’t stop leaking after trying to bleed radiator

Hi, I tried to bleed this radiator but after doing so I can’t seem to tighten it enough to stop this dripping.

If I tighten it where a flat head screwdriver would fit in the pic, it gets to a certain tightness but water still drips, or if I try tighten anymore it seems to come loose and water starts spraying until it’s tightened a bit again.

How do I fully tighten / stop the dripping anyone know please?