It is one of the strange things about slow cookers - root veg take longer to cook than meat. Apparently it's because it reaches cooking temperature for meat some time before reaching those for veg. You must:
cut the veg into thin slices and
put them in the hottest part of the pot ie next to the element and
ensure they are fully immersed in the liquid.
You can also parboil or saute before adding to the pot.
HTH