When you do a survey, you will frequently be disqualified and then taken back to the page that asks if you want to try another survey. Obviously, you don't get credit for surveys you could not complete.
But when you work through a survey and get to the page that says you've completed the survey (with or without congradulations), then you just have to wait until the survey is credited to your account.
I usually keep the final page of the survey open until I receive credit in my account. I also keep my account page open and refresh it periodically until I receive credit. Credit will usually come through in 5-10 minutes.
If it doesn't come through in that time, something happened with the transmission of information and it's unlikely that you will get credit. Just chalk it up to experience and move on. (Or you can send a Trouble Ticket to FC admin and at least notify them of the problem. They will tell you a)to wait a few days b)to wait 30 days c) there is nothing we can do.)
If this happens frequently to you, make sure you clear your temporary file/cookies before you start the survey process. I know that is important when doing regular offers. Not sure whether it helps with surveys, but it will improve your PC operations.
Offers generally take longer to credit and you just have keep checking back on your account page to see if you've credit.