Was it for sure Liquid White, and not White Gesso, that you used? If it is indeed liquid white, and it's getting gummy, it may be drying too quickly, or not enough used. The canvas you have may be a cheaper one that is absorbing your liquid white too quickly, so you could double prime it yourself with White Gesso first. Give the liquid white a good stir with a mixing stick before applying to ensure the oil is well distributed in the paint, too. Hope this helps, happy painting!