That has happened to me as well./....Try not to get too discouraged.....all and all I probably don't get credit for about 5 out of every 100 offers I do....That's about a 95% success rate....I am happy with that.....make sure you clear your cookies out regularly (that will help)
Plus it also helps to leave the success page up after you complete an offer until you get the credit.
Some offers get rerouted for some reason and you end up on the Vindale site.....that's not FC's fault.....its just routed wrong....it happens but try to stay positive.....you can make good money here on FC
I make over $100 every month here and love it....Good luck in the future