They are still young and may not want to take veggies just yet. However, you should still offer it everyday, no if's and's or but's

When they are young they are more carnivorous but that shouldnt stop you from offering it like I said above. If anything, try different types of veggies, you told me you feed them lettuce right? Lettuce is good but lettuce alone isnt going to cut it, you have to provide a balanced diet with a variety of different veggies. Try stuff like basil, culantro, kale, dandelion leaves, raddichios and aquatic plants especially since it is more natural, stuff like anacharis, horn wort, duck weed are good, though there are more to pick from.
You can always dip the lettuce or whatever you want to feed them in something smelly like tuna or catfood and then offer it to them. The strong fishy smell will maybe make them eat it. A lot of people have used this trick, including me and it works.