First: do you clear your cookies and temp BEFORE you start doing surveys for a given survey site?
Whether you qualify depends on your demographics and whether they match the specs of the survey. Usually you will go thru the early questions and get DQ's (disqualified) quickly. I'm retired (>65) and I find that I usually get DQ'd 10 times before I finally get to do a survey to the end.
Once I've completed the survey, I do everything I can to ensure that I get credit. You need to keep the congrads page open for at least 10 minutes or until you see the credit in your account. I usually have my account page open on another tab so it's easy for me to click on that tab, refresh it, check the status.
If it hasn't credited in 10 minutes, I then take a screen shot of the congrad's page, and paste in onto my Paint software; I add comments to identify the survey (what subject, date/time completed; survey site; credit expected). Then I save it to a jpeg picture to my shared pictures library; name it so it's easy to identify later.
If I haven't gotten credit in a couple of days, I then send a ticket to Support, attach the jpeg picture as proof of completion and ask them to check it out. You may still not get credit, but it does give FC something to work with.
I do not go on to another offer or survey until I've received credit or 10 minutes have past and I've completed the jpeg picture. Since you should clear your cookies and temps BEFORE you go onto another survey site or another offer, if you do clear your cookies too quickly, you may clear out the means for the survey company to get the information they need to confirm that you took the survey and get the credit back to FC.
On regular offers, you will not receive credit immediately, so it is even more important for you to wait at least 10 minutes before you go onto another offer. Then clear your temps and cookie and proceed to the next offer. If you decide not to clear your cookies and temps, you run the change that what you have in your cache will cause the offer to not to credit.
One last thing: you can only complete one survey from a given survey site every 24 hours. So when you complete a survey, take note of the time and do not attempt another survey from that site until at least 24 hrs pass.