There are several reasons!
1) The most typical reason is that you might have completed the same offers before somewhere else. This happens frequently to those who have used other reward apps. Did you complete the same offer from a different offer wall like Supersonic? It doesn't matter! As long as the offer was from the same advertiser, you won't get rewarded again. Never!
2) Another typical reason is that you made some mistakes! For example, if you didn't use your real name, address, email or contact number, you won't get rewarded. The advertisers have their own systems to cross- check your input was true or not. So please do it correctly!
3) The last typical reason is that you didn't complete all requirements. Some offers ask you to sign up their services. Some game apps require you to reach a certain level or play more than 30 min or so. Please read your mission carefully.