Dry food isn't that important - their diet is (if I remember correctly) 80% hay, 15% veg and 5% dry food, so don't worry if they aren't eating that much.
Do you feed a mix or pellets/nuggets? If you are feeding a mix then they probably aren't eating much because they are selectively feeding, they are just eating their favourite bits. Just like if you gave someone a plate full of chocolate and veg, they're more likely to eat the chocolate. You could try changing them (slowly) to a pellet food - like Burgess Supa Guinea Excel, Science Selective or Pets at Home nuggets. You need to mix their old food with the new for a week or so, slowly increasing the amount of new food and decreasing the amount of old food - if you suddenly change it it could upset them. Some guinea pigs don't like the pelleted foods but if they will eat it it is a lot healthier because they are getting all the nutrition in one nugget, they don't have to eat the whole mix for it to be balanced.
I wouldn't cut down on the amount of vegetables you are feeding, but maybe you could spread it out into two meals? They could have one in the morning and one in the evening?