Unfortunately getting credit on offers is an iffy situation.
1. You need to clear your cookies and temp files BEFORE you start an offer. The advertiser (somehow) can check your cookies and if you have a record on your PC that you took, tried, looked at that offer or a similar offer from the same advertiser, odds are you will not get credit.
2. Check the requirements of the offers and follow them to the letter. If you need to provide your email address, use an email address that is new or one you set up for offers. The purpose here is to make sure that confirmation emails get to that email and you know where to look for them. Check your spam for the confirmation emails if you don't see them in your mail box.
3. Take a screen shot of the page that says you've completed the offer. If you don't get one, odds are you haven't completed the offer yet. (Some offers are long and involved, some are short and sweet.)
Save that screen shot to your PAINT software, add a few comments to describe the offer (date, time completed; amount of credit due; type of offer -- what ever you consider relevant.)
Save the Paint page to a jpeg window in your Shared Picture library. Label it so you can easily identify it later (ie OfferNamePOCdate.jpeg)
4. Leave the offer completion page open for at least 10 minutes before you go on to another offer. The systems need some time to record the transaction between the advertiser and FC. Closing the window too soon and clearing your cookies and temps to go on to another offer may easily wipe out the communications needed to record the transaction and get you your credit.
5. If the offer hasn't credited in a reasonable amount of time (varies from offer to offer), send in a support ticket and attach the jpeg picture as proof that you completed the offer. No guarantee that this will result in your getting credit, but it does give FC something to work with.