Some cruciferous vegetables e.g. cabbage and brocolli can cause cramps and gas. You could try add a piece of juiced ginger to help with cramps or bloating.
Also, I sometimes experience problems with juicing; not with my liver but with bloating, which I think my be linked to leaky gut caused by candida. From what I can remember reading leaky gut can cause the things you eat to enter into your blood from your stomach and cause problems. I did a juice fast for three days and had bloating but by the third day the bloating had gone, but returned when I started to eat "normal" food.
You could try using different fruit/vegetables on rotation