350 followers on pixiv is quite small number. How many posts you have there?
as i noticed number of followers mostly depends on how long you are there, you constantly gain followers every day
pixiv will promote your art by itself if you add right tags, so maybe you forgot to add tags? or you used some unknown tags?
Without tags nobody will see your work and you will not get any followers, on the other hand I suspect that when you gain followers pixiv will start checking if your pictures are properly censored.