Ok I just joined and a little confused.

I joined on monday and did a greenfield survey. I got credit for it. Yesterday I did another Greenfield but never got credit. So I read about clearing cookies so this morning I cleared my cookies and did another Greenfield survey. I qualified it took me a good 30 min (maybe more) to take the survey, there was a ton of commercials to watch and a bunch of questions. I get to the end and it says thanks for you opnion but you didnt qualify. How do I not qualify I just took the entire survey, they got all my answers but arent going to pay? Does this happen a lot. I could see if I answer the first 5 or so question and didnt qualify but this one said I did made me do all the work then nothing.

Also what is the deal on clearing cookies? Do I do it after each survey? Do I have to wait for it to credit before I clear...help?