Non-payment for Services

I have completed online surveys with this company for many years. They do offer lots of paid surveys, and they do pay quickly. The problem is, there have been many occasions when I complete the survey, reach the end, "Thank you" page, and then am redirected back to the Opinion Outpost site to receive credit, only to be told the survey has been closed. This happened early this morning (about 12:20 am), with a $7.50 survey. I spent a lot of time completing this survey, answering the questions carefully and honestly, only to be told that I would not receive the payment I was promised.