Very fussy towards cat food

This problem started about a month ago and we have tried everything. My cat's are very fussy towards what cat foods they eat.
We have recently been giving them whiskas which they will eat but then all of a sudden go off it and want a different food.
We have tried Whiskas, Kitekat and Sheba with them and they will eat them for a while and then go off them after a bit.
When we leave the bowls outside because the cats don't eat it they still won't even eat it and it will be left there in the morning. They come in in the morning demanding more food when there is already some out there. Can anyone tell me why this is?

My cats are like that too, they are very fussy. The only thing I can get them to eat is Fancy Feast shredded, different varieties, they love it. I also, try them on the more healthy foods but they just refuse to it. I end up throwing it out. I have two cats that have problems with UTI and they refuse to eat any of the prescription foods, so all I can do is give them what they want and mix it with water.

My kitchen looks like the cat food aisle of a supermarket! I have every flavor of FF, plus a whole bunch of others, plus various 'crunchies', plus whatever else on a given day. Cats get bored easily (it's not like they have a riotous life), so the least we can do is give them something different to eat as often as possible (mine never get the same thing twice in a row - anything but - except for Temptations and they're all mixed too.