Some reasons may be:
A) The offer crediting may be delayed and you can check back in 24-48 hours.
B) You already have an existing account on the advertiser website.
C) You did not complete the offer fully and exited the website prematurely.
D) You did not follow the offer description/instructions.
E) Your adblock software is preventing you from completing the offer.
F) Your browser is set to not accept cookies.
You can try to complete the offer again if you have not been credited for it.