I was having this issue with my older kitty. She's 15 and I figured it was just age. I got her a specific calorie inducing gel from PetsMart that I put on her paw to lick off. It helped really - she would eat well when I gave her that. HOWEVER - I also took her to the vet and they gave her an antibiotic shot and she's a brand new kitty. Come to find out - she's had a cold for the past few months and now feels MUCH better.
So - try the Calorie inducing gel stuff at PetsMart but also talk to your vet about the possibility that your cat may just not be feeling well.