It's possible that your account has been red flagged. I've heard of this happening when people all of a sudden change gender, age, race, etc. in order to qualify for a particular survey.
I don't do surveys much any more, but when I do, I'm always an old white guy.
It's also possible that they monitor how long folk spend on a survey. Rushing and fairly obviously christmas treeing the answers wouldn't be a good idea.
Not saying either of these is your particular problem. Just informing.