There are several veggies you can try. Mine generally love Kale/Collards/Mustard greens all mixed up. I sometimes throw in some fresh Swiss Chard. Lettuce is not that great for them, as it has little or no nutritional value.
My 2 year olds don't seem to eat as much in the way of veggies as they used to and go in spurts with eating crix. I firmly believe that if they need it, they'll know to eat it. Just keep offering fresh food daily. Unless you start to see shrinking fat pads (the "pillows" on top of the head behind the eyes) or notice a dramatic weight loss, you can put more stress on your beardie by trying too hard to get them to eat.