Hi B_S, Speedy and B_B,
I just thought up an idea after reading the theory B_S came up with in the paint possibly skinning during prolonged storage.
Shake the can in various directions and angles for at least two minutes to give the marbles inside a chance to break up the skin as much as practicable and mix it into the paint. Then put it on the shelf again for a day or two to give the skin fragments a chance to possibly dissolve or at least soften enough to pass through the outlet easily together with heating the can.
