number one- you want the mold to be HOT. investment molds are usually pored fresh from the furnace after melting out the wax.
number two- Is your mold VENTED? you are not pouring molten gold into an Empty mold, you are pouring it into a mold that is already filled - with air.
you need to make provision in your mold for the air to come out as the metal flows in. A GATE for metal to flow in. And one or more VENTS that the air can escape thru.
Investment allows the gas that comes out of the metal as it cools to escape thru the mold wall… but its not porous enough to allow the air to pass thru as fast as you need the metal to flow in,
number Three- for small castings you need a centrifugal casting machine to place a pressure head in the metal so that it will move quickly into the mold.
They make small ,spring wound centrifugal casting machines you can clamp to a desktop and that will handle ring sized molds.