Does it really say "
curves do not curve correctly" or does it say "
curves do not cross correctly"?
That is usually caused because the splines are not attached to each other. Make sure that the points shared by two splines are welded (select the intersection and if there is more than 1 point selected, hit CTRL+w to weld them together).
It is also caused when you don't select the splines in the right order.
About having to flip, it depends on what direction you select the splines (clockwise or counter clockwise). There isn't a real standard since it depends on your viewport and other things of the sort.