Why isnt my guinea pig eating fruits or veggies?

This is my third guinea pig and I know which fruits/veggies are hazardous to guinea pigs but this one worries me. Usually if you put romaine lettuce or a carrot in front of them they are suppose to pig out on it. This one barely nibbles on it or turns his head and it makes me sad. Is there a way that he can start eating veggies and fruits?