What should i do about my cat not finishing her food? should i give her something different?

My cat is 5 months now and at first, she would finish her food for everytime I fed her. I know since she is getting older, I would have to feed her more. However, for the last two weeks, she wouldnt even finish her food. Perhaps thats a sign to give her something different but if it is, whats the best brand for cat foods? Ive been feeding her Whiskas, Friskies (canned foods) and Purina (dry foods) since I got her.