Does more than one person play from your ip address? Did you try to access lots of surveys daily? Did you fail to update your personal information? Did you ever give conflicting information from survey to survey, even unintentionally?
While I have never been banned from the surveys, I did go through a period when I did a lot of them and they pretty much disappeared for a few days. When I backed off of trying to do 8 or 10 a day and changed to videos for most of my credits, I found I could get 1 or 2 a day. With today's WGT, getting credits from the mobile app seems the easiest way to stay ahead.