Your mincer may be absolutely fine, maybe you will just have to put the meat through a couple of extra times using smaller holed discs. We bought ours because it has an attachment that takes the sausage skins, they can be stretched over the mouth, rather than trying to squeeze it all into the skins with a teaspoon.
Kohlrabi needs loads of water, mamie. If it doesn't get it, they are small and hard, as you say, so you then have to eat them when they are very small, maybe between ping pong and tennis ball size, before they get a chance to dry out too much.