Maybe you didn't leave them in long enough?

I've roasted lots of things.....yams, potatoes, carrots, rutabega,
beets, leeks, onion.....at about 425 for 45 minutes to 1 hour, and they
sure sweeten up for me! All I've given them is a drizzle of olive oil.
They get parts that are brown and carmelized, crispy on the outside but
the inside is soft and sweet. What did you try for temperature and
roasting time?