Focus Groups... How Frequent?

I have been trying my hand at doing focus groups. However, it seems that out of 20 I sign up for, only 1 call me back.

Now that I've done 1 recently, am I supposed to lie about when my most recent focus group was? There is a common question on the pre-req surveys that ask "When was your most recent focus group?" and whenever I choose "Last 6 months" the survey ends.

How do you increase your odds of getting picked?