Some offers do take time to credit. Some won't.
It very important that you clear you cookies and temp file before you begin an offer. It is also worthwhile to wait several minutes after you have completed an offer to allow the communications process (between you, the advertiser and FC) to be completed.
If you have to respond to a offer completion email, do that before you close the final page of the offer. You usually have to provide your email address and the offer site will send a confirmation email to you to make sure your email address is true. If they don't get a confirmation from you that you received their email, then they will think the email address is bogus and not give you the credit. Many people set up a separate email address for offers so their main email address isn't overwhelmed by spam. Plus it's easier to see the confirmation email request in the new address mailbox.
After you've completed the basic confirmation, if you still haven't received credit, take a screen shot of the final page of the offer, paste it to PAINT, add some comments at the bottom that will remind you of what the offer is; what you did and what you expect. Then save it as a jpeg picture for future reference (I would title the picture: "FCofferDate" to make it easy to find. I save mine to shared pictures.
If you haven't received credit in a few days, send a TT to support and attach the jpeg picture to give FC info to track it down and hopefully get you and FC credit.