Paint might be old, or might just be shit, but you can look into buying paint mediums. I've got Liquitex flow-aid, and velijo paint thinner that will work better then just water.
What I also find helpful is quickly dabbing a brush in water, then picking up paint, then running the brush against something, wastes a little paint but helps to get the point on the tip, and prevent overloading the brush. And you can kinda gauge if it's still to thick, dab in water and streak it again until you think it looks good.
But again, sometimes paints are just shit, and your better off finding an alternative. Like some of the vellijo metallics are gummy and blow balls, but their airbrush ones work perfectly fine to brush on, etc.