I completed the offer in the Rise of kingdom then why I haven't received my reward.
I had the same problem ( i assume you're talking about TapJoy),
There seems to be a problem when it comes to handing out rewards upon completing offers.
So the best thing you can do now is to wait for 24 hours untill you get your reward.
Because the process is not instantaneous, and if you dont get your reward after that > file a complaint ( Ticket ) and wait for their responce.
Usually you wait for 3-4 days untill they review your complaint and offer a solution
(gems or no reward at all)
Also leave your tapjoy running in the background while you're completing your tasks, that way you should be able to avoid some unwanted bugs like
(you completed offer and tapjoy didnt register)
And another thing ( this has nothing to do with IGG ) so title should be changed !