poly s paints

i am back into plastic model building after a loooooong spell. i used to use poly s paints for my tank models with good results. i am currently working on a 1/87 german locomotive, molded in red and black. i am having trouble covering the black with red paint. it seems the red is too thin. i have BBs in the bottle, shake it and have a stirer (too many gadgets but that is another story) and it stil doesn't seem to thicken up. i may switch back to thinner bases paint if i can't solve this.