Guinea pigs learn from their elders what is safe to eat and what not - by watching them eat, by smelling their mouths and by snatching food from their mouths. It is always so much harder for adult piggies on a no or very limited veg diet to start eating veg - or even learn to eat hay... They also learn to drink from a bottle or a bowl from their elders. Without that guidance, it is a lot harder.
You may find this article series about the development from birth to death interesting as it may help you to understand better what you are up against:
Journey through a Lifetime: The Ages of Guinea Pigs
Be patient and persist. My Dylan spent the first two years of his life in a hamster cage being only fed lettuce and carrots; he wasn't able to walk a yard without his back end breaking down. It took months for him to get fully onto a normal diet and to eat peppers (thanks to his young companions - due to social deficits he initially didn't work out with adults) and to learn to walk and run - but he did get there and lived another happy four years to the age of 6 years... He was a lovely natured and loving gent, nd is much missed. His two widows are still with me.