Welcome to the forum.
At least on my Hayward there is a circular piece of soft gasket material inside the cap to butt and seal against the spout edge when you tighten it down. If this is missing or damaged it will leak. Can go to an auto parts store and but a small sheet of some soft gasket material and cut a new one. Since you said you tried two different caps I doubt this is the problem but it's my best guess for now.
Al