When the paint doesn't flow into the felt tip, you just have to shake the marker body and listen to the inner ball rattle and generate movement for the paint to flow again into the felt tip. Make sure that you close the lid tightly so that the paint and the felt tip pin doesn't dry and clog up. Cleaning the felt tip after use w/ tissue also helps to lessen paint build up. Some go so far as to tape up the lid so that air can't get in the cap. If the felt tip dries up, sometimes it is works to dip the tip in thinner to clean up the dried paint.
For more scale model paints, kindly refer to this post. Paint conversion tables can also be found here.
