My two used to do this and I actually made the mistake of buying 2 boxes as it was buy one get one free deal and half way through the 1st box of Whiska's, they decided they didn't want to eat it anymore! I had one full box of Whiskas left over, which I had to give to my friend, whose cats weren't that fussy! (Lucky Cow!)
I tried to persevere, I was determined to finish the old box but gave up as they made me feel guilty and wouldn't eat it! I eventually gave the rest to my friend again, who was very grateful as she didn't need to buy any food for a while!