Some reasons may be:
A) You already have an existing account on the advertiser website or have completed this offer already.
B) You did not complete the offer fully and exited the website prematurely.
C) You did not follow the offer description/instructions.
D) Your adblock software is preventing you from completing the offer.
E) Your browser is set to not accept cookies.
F) The offer points are pending for 30 days, please check https://www.prizerebel.com/offers_completed.php
You will have to contact the specific offerwall regarding the missing credits for an offer you have completed if it is not listed as pending. Please click on the offerwall and look for the support or contact section to submit a ticket.