Do you par boil them before you put the in the oven? Are you cutting them into equal sizes? If some are smaller/thinner than others they will burn before other bits are cooked. What temperature do you cook them at?
I boil mine for about 8 min, then drain, fluff about a bit in the colander to roughen the edges up, spray with Fry Light and roast for about 40min at 160C (fan) turning once or twice.
If you have no luck the best solution is to cook them covered in foil until they are tender, then take the foil off and leave them to crisp up for 10 min or however long. I do this all the time with veg that are quite tough such as swede, or when I am cooking stuff covered in spices that would burn otherwise.