Wheatgrass is one of the few things that the X-1 Mini doesn’t really work well with. It works ok if it’s just a small amount of wheatgrass combined with another recipe, but if it’s straight wheatgrass or mostly wheatgrass, the strands of grass end up clumping up into a ball in the grinder and won’t go through. I recommend buying one of the standard wheatgrass juicers, as it will give you much better results.
2020 Update: The Goodnature M-1 works very well on wheatgrass. The grinder is very similar to the X-1 mini, however the fact that it’s slightly smaller on the M-1 makes it work better with wheatgrass.